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Field-based quantification of soil greenhouse gas emissions from the Philippines’ agriculture sector is missing for vegetable production systems, despite its substantial contribution to agricultural production. We quantified soil N2O emission, CH4 uptake, and CO2 efflux in vegetable farms and compared these to the secondary forest. Measurements were conducted for 13 months in 10 smallholder farms and nine forest plots on Andosol soil in Leyte, Philippines using static chambers. Soil N2O and CO2 emissions were higher, whereas CH4 uptake was lower in the vegetable farms than in the forest. Vegetable farms had annual fluxes of 12.7 ± 2.6 kg N2O-N ha−1 yr−1, −1.1 ± 0.2 kg CH4-C ha−1 yr−1, and 11.7 ± 0.7 Mg CO2-C ha−1 yr−1, whereas the forest had 0.10 ± 0.02 kg N2O-N ha ha−1 yr−1, −2.0 ± 0.2 kg CH4-C ha−1 yr−1, and 8.2 ± 0.7 Mg CO2-C ha−1 yr−1. Long-term high N fertilization rates in vegetable farms resulted in large soil mineral N levels, dominated by NO3– in the topsoil and down to 1-m depth, leading to high soil N2O emissions. Increased soil bulk density in the vegetable farms probably increased anaerobic microsites during the wet season and reduced CH4 diffusion from the atmosphere into the soil, resulting in decreased soil CH4 uptake. High soil CO2 emissions from the vegetable farms suggested decomposition of labile organic matter, possibly facilitated by plowing and large N fertilization rates. The global warming potential of these vegetable farms was 31 ± 2.7 Mg CO2-eq ha−1 yr−1 (100-year time frame).

Land-use conversion accounts for various modifications in the scope of physical, and geochemical properties of associated components, especially soil properties. Agroforestry is a type of land management system where forestry and agricultural practices are incorporated together to achieve certain land-use management goals with respect to various environmental factors. The microclimatic condition of forest and agroforestry has a definite situation that has the potential to alternate with the change of management practices. Our study aimed to measure the soil’s physical and biogeochemical properties, greenhouse gases (GHG) potential, and nitrogen (N2) fixation properties of the respective sites during the respective study period. A certain alteration of nitrogen availability through fixation, soil organic carbon due to the availability of a thick debris layer in the forest was observed whereas water-filled pore space varied in different agroforestry and forest sites was determined because of compaction effects of agricultural practices. The presence of trees in the study area showed the trends of resulting differentiation in N2 fixation and CH4 emission. Soil microbial biomass created an impact on releasing GHG which is altered by surrounding environmental factors.

Soil degradation is expected to continue as forest conversion into other land uses increases significantly. In Indonesia, Jambi is one of the main areas for the development of oil palm and rubber, whichare mainly converted from the forest. As a base for better management, we attempted to study macro-porosity in rubber and oil palm plantation, in comparison to secondary forests. Four landuse systems (secondary forest, jungle rubber, rubber plantation and oil palm plantations) in Bukit Duabelas, Sarolangun District, Jambi Province, Sumatera, were selected for this study. The number of macropores in vertical or horizontal planes and their related factors (root mass, litter thickness, % organic C, bulk density, water content at pF 0 and pF 2.54, aggregate stability) were measured within the soil profiles. Forest conversion to jungle rubber, rubber and oil palm plantation led to a decrease of macro-porosity in the soil profile, especially in the upper 50 cm. Macropores, both at vertical and horizontal planes in the secondary forest was significantly higher than other landuses. Horizontal macropores in jungle rubber were higher than rubber and oil palm plantation, but not the vertical macropores. Among the soil properties measured, litter thickness, coarse root dry mass (Ø >2 mm), mesopores and aggregate stability were closely associated with soil macro-porosity. However, macro-porosity in the soil profile was insignificantly correlated to soil bulk density and % organic C. Increasing the number of horizontal macropores resulted in higher nutrient leaching, especially K and Na.

Intensive management practices in large-scale oil palm plantations can slow down nutrient cycling and alter other soil functions. Thus, there is a need to reduce management intensity without sacrificing productivity. The aim of our study was to investigate the effect of management practices on gross rates of soil N cycling and soil fertility. In Jambi province, Indonesia, we established a management experiment in a large-scale oil palm plantation to compare conventional practices (i.e. high fertilization rates and herbicide weeding) with reduced management intensity (i.e. reduced fertilization rates and mechanical weeding). Also, we compared the typical management zones characterizing large-scale plantations: palm circle, inter-row and frond-stacked area. After 1.5 years of this experiment, reduced and conventional management showed comparable gross soil N cycling rates; however, there were stark differences among management zones. The frond-stacked area had higher soil N cycling rates and soil fertility (high microbial biomass, extractable C, soil organic C, extractable organic N, total N and low bulk density) than inter-row and palm circle (all p ≤ 0.05). Microbial biomass was the main driver of the soil N cycle, attested by its high correlation with gross N-cycling rates (r = 0.93–0.95, p

Efficient use of nutrients is a key requisite for a sustainable intensification of agriculture in order to meet the increasing global crop demand while minimizing deleterious environmental impacts. Agroforestry systems exhibit tree–crop interactions, which potentially contribute to nutrient-efficient agro-ecosystems. Our goal was to determine whether the conversion from cropland monocultures to alley-cropping agroforestry increases nutrient response efficiency (NRE), the ability of plants to convert available nutrients into biomass. We found that crop yield, plant-available nutrients and NRE were comparable between agroforestry and monocultures, but the trees in agroforestry had high NRE, contributing to nutrient retention of the agroforestry systems as a whole. The unimodal relationship of the crops’ NRE with plant-available nutrients suggests that NRE values were beyond optimum in both agroforestry and monoculture indicating nutrient saturation. This indicates that fertilizer inputs can be reduced (or optimized) without sacrificing crop yield or profit. Based on the NRE curves, we assessed that a reduction of plant-available N by 50% would lead to a decrease in crop yield by 17% and a concomitant increase in N response efficiency by 67%, whereas a similar reduction of plant-available P would lead to a decrease in crop yield by 8% with an increase in P response efficiency by 83%. An optimized fertilization to achieve such lower levels of plant-available nutrients will have beneficial effects on nutrient retention and redistribution. Optimizing fertilizer input will make alley-cropping agroforestry a productive and profitable agro-ecosystem that contributes to an ecologically sustainable agriculture.

Soils under natural, tropical forests provide essential ecosystem services that have been shaped by long-term soil–vegetation feedbacks. However, deforestation of tropical forest, with a net rate of 5.5 million hectares annually in 2010–2015, profoundly impacts soil properties and functions. Reforestation is also prominent in the tropics, again altering the state and functioning of the underlying soils. In this Review, we discuss the substantial changes in dynamic soil properties following deforestation and during reforestation. Changes associated with deforestation continue for decades after forest clearing eventually extend to deep subsoils and strongly affect soil functions, including nutrient storage and recycling, carbon storage and greenhouse gas emissions, erosion resistance and water storage, drainage and filtration. Reforestation reverses many of the effects of deforestation, mainly in the topsoil, but such restoration can take decades and the resulting soil properties still deviate from those under natural forests. Improved management of soil organic matter in converted land uses can moderate or reduce the ecologically deleterious effects of deforestation on soils. We emphasize the importance of soil knowledge not only in cross-disciplinary research on deforestation and reforestation but also in developing effective incentives and policies to reduce deforestation. Deforestation and reforestation cause dramatic changes to tropical ecosystems, including underlying soil properties and their corresponding ecosystem services. In this Review, the impacts of this land-use change on soils and their functions are discussed.

Experimental evidence of nutrient limitations on primary productivity in Afrotropical forests is rare and globally underrepresented yet are crucial for understanding constraints to terrestrial carbon uptake. In an ecosystem-scale nutrient manipulation experiment, we assessed the early responses of tree growth rates among different tree sizes, taxonomic species, and at a community level in a humid tropical forest in Uganda. Following a full factorial design, we established 32 (eight treatments × four replicates) experimental plots of 40 × 40 m each. We added nitrogen (N), phosphorus (P), potassium (K), their combinations (NP, NK, PK, and NPK), and control at the rates of 125 kg N ha

Fine roots represent a small but important part of belowground plant biomass, however, field-based evidence of how nutrient availability control fine root production in species-rich tropical forests is scarce yet remain imperative to our understanding of ecosystem biogeochemistry.To evaluate the responses of fine root production and plant-available soil nutrients to N, P and K fertilization thereby identifying which (if any) nutrients limit plant growth and microbial processes, we conducted a large-scale, full factorial nutrient manipulation experiment (8 treatments × 4 replicates: 32 plots of 40 × 40 m each) in a humid tropical forest in Uganda. We added nitrogen (N), phosphorus (P), potassium (K), their combinations (NP, NK, PK, and NPK) and control at the rates of 125 kg N ha−2 yr−1, 50 kg P ha−2 yr−1 and 50 kg K ha−2 yr−1, divided into four equal applications. We quantified fine root biomass (0−10 cm soil depth) at the end of the first and second years of the experiment by excavating soil monoliths (20 cm × 20 cm) at six random locations within each plot. Fine root production in the top 30 cm soil depth was estimated using the sequential coring technique in the second year of the experiment.It was determined that the addition of N reduced fine root biomass (FRB) by 35% after the first year of the experiment and did not change in the second year whereas K addition was associated with reduced fine root production, suggestive of an alleviated ecosystem-scale N and K limitation. This rapid reduction in fine root biomass and production highlight that maintaining a large fine root network is an energy and resource-intensive process, therefore, trees will scale back their root network when they have adequate resources available. Next, a strong positive relationship was evident between FRB and NH4:NO3 ratio and highlights how FRB decreases dramatically when NO3 concentrations surpass NH4 concentrations (NH4:NO3 < 1). Additionally, nutrient additions resulted in a cascade of biochemical responses in soil nutrient availability. Specifically, (1) the interaction effects of all three nutrients (N, P and K) enhanced net N mineralization and nitrification rates. This highlights the complementary roles of these nutrients in regulating soil processes related to N-cycling in this ecosystem. (2) Microbial biomass C increased with P additions but was dependent on the season. Lastly, P additions increased plant-available P by 80%. This large increase could indicate that the demand for P was not very high. Our data show that N and K are particularly important in regulating fine root growth in this ecosystem.

Conventional intensive management, such as high fertilizer and herbicide applications, are common practice in large-scale oil palm plantations. One of the proposed solutions to lessen its environmental impact is to reduce fertilization and employ mechanical weeding without sacrificing yield and profit. A full factorial experiment with two fertilization rates (260 N, 50 P, 220 K kg ha-1 yr-1 as conventional practice, and 136 N, 17 P, 187 K kg ha-1 yr-1, equal to harvest export, as reduced management) and two weeding methods (conventional herbicide application, and mechanical weeding as reduced management) was established in 2016 at a large-scale oil palm plantation (planted in 1998-2002) on a sandy clay loam Acrisol soil in Jambi, Indonesia. Soil CO2, N2O, and CH4 fluxes were measured monthly from July 2019 to June 2020, using vented static chambers. At each plot, the measurements were conducted on two randomly selected subplots, and in each subplot, we measured at three management zones (palm circle, inter-row, and frond-stacked area). During 2017-2020, fruit yield did not differ among treatments (fertilization: P=0.35; weeding control: P=0.11). Soil CO2, N2O, and CH4 fluxes also did not differ among treatments (fertilization: P>0.81; weeding control: P>0.28). Area-weighted from the three management zones, soil CO2 fluxes (mg C m-2 h-1) were 61±2 for conventional and 65±4 for reduced fertilization and 64±4 for herbicide and 62±2 for mechanical weeding. Soil N2O fluxes (µg N m-2 h-1) were 46±12 for conventional and 45±16 for reduced fertilization and 57±15 for herbicide and 34±12 for mechanical weeding. Soil CH4 fluxes (µg C m-2 h-1) were -17±2 for conventional and -17±3 for reduced fertilization and -17±3 for herbicide and -17±2 for mechanical weeding. Distinct differences were observed among the three management zones. Frond-stacked area, with high soil organic carbon and low soil bulk density, had the highest soil CO2 emission and soil CH4 uptake (P≤0.01). Palm circle, with fertilizer application and high soil bulk density, had the highest soil N2O emission and lowest soil CH4 uptake (P≤0.01). Inter-row area, with low soil organic carbon and no direct fertilizer application, had the lowest soil CO2 and N2O emission (P≤0.01). Soil CO2 (rho=0.64, P≤0.05) and N2O (rho=0.53, P≤0.05) fluxes were positively correlated with total mineral N. Soil CH4 flux was negatively correlated with total mineral N (rho=-0.30, P≤0.05) and positively correlated with water-filled pore space (rho=0.66, P≤0.05). Although the frond-stacked area only accounted for 15% of the oil palm plantation area, it contributed 30% of soil CO2 emission and 41% of soil CH4 uptake. The palm circle accounted for 18% of the oil palm plantation area and contributed 79% of soil N2O emissions. Our results indicated that the inherent management zones in oil palm plantations should be spatially represented for accurate quantification of soil greenhouse gas fluxes. Our findings showed that reduced management maintained yield whereas soil greenhouse gas fluxes remained high at least during the 3-4 years of this management experiment, which signified the legacy effect of more than a decade-long conventional management in this mature oil palm plantation.

Special thanks to our field assistants in Indonesia (Basri, Bayu and Darwis) and to Frank Tiedemann, Edgar Tunsch, Dietmar Fellert and Malte Puhan for technical assistance. We thank PTPN VI and the owner of the plantation at Pompa Air for allowing us to conduct our research at their plantation. We would also like to thank the Spanish national project GEISpain (CGL2014-52838-C2-1-R) and the DAAD (scholarship from the programme ‘Research Stays for University Academics and Scientist 2018, ref. no. 91687130)' for partly financing A. Meijide during the preparation of this paper., The potential of palm-oil biofuels to reduce greenhouse gas (GHG) emissions compared with fossil fuels is increasingly questioned. So far, no measurement-based GHG budgets were available, and plantation age was ignored in Life Cycle Analyses (LCA). Here, we conduct LCA based on measured CO2, CH4 and N2O fluxes in young and mature Indonesian oil palm plantations. CO2 dominates the on-site GHG budgets. The young plantation is a carbon source (1012 ± 51 gC m−2 yr−1), the mature plantation a sink (−754 ± 38 gC m−2 yr−1). LCA considering the measured fluxes shows higher GHG emissions for palm-oil biodiesel than traditional LCA assuming carbon neutrality. Plantation rotation-cycle extension and earlier-yielding varieties potentially decrease GHG emissions. Due to the high emissions associated with forest conversion to oil palm, our results indicate that only biodiesel from second rotation-cycle plantations or plantations established on degraded land has the potential for pronounced GHG emission savings., This study was financed by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)— Project-ID 192626868—in the framework of the collaborative German-Indonesian research project CRC990 (subprojects A03, A04 and A05)., Spanish national project GEISpain (CGL2014-52838-C2-1-R) and the DAAD (scholarship from the programme ‘Research Stays for University Academics and Scientist 2018, ref. no. 91687130)

In this study, we report the impacts woodland savanna burning has on the soil trace gas fluxes in seasonally dry woodland savannas in Burkina Faso. We measured nitrous oxide (N2O), nitric oxide (NO), carbon dioxide (CO2) and methane (CH4) fluxes at two ongoing long-term experimental sites (Tiogo and Dinderesso), each included three fire-exclusion plots and four annually burnt plots (0.24–0.25 ha plot size). We measured soil trace gas fluxes using chamber methods before and after fire during the dry season and at the start of and during the wet season. There was considerable volatilization of aboveground biomass C and N during fire, but soil N2O, NO, CO2 and CH4 fluxes were not significantly influenced following fire events. Instead, soil moisture dynamics were more important in regulating soil CO2 and N2O emissions. The onset of the wet season induced a short-term pulse of N2O emissions, when water-filled pore space exceeded 60%. We also detected a short-term pulse of NO emissions, lasting only four hours, following fire. The low soil N-oxide fluxes reflected the sites’ extremely low inorganic N levels in the soil.

Tropical forests contribute about one third to global annual CH4 uptake by soils. Understanding the factors that control the soil‐atmosphere exchange of CH4 at a large scale is a critical step to improve the CH4 flux estimate for tropical soils, which is presently poorly constrained. Since tropical forest degradation often involves shifts in nutrient availabilities, it is critical to evaluate how this will affect soil CH4 flux. Here, we report how nitrogen (N; 50 kg N ha−1 yr−1), phosphorus (P; 10 kg P ha−1 yr−1), and combined N + P additions affect soil CH4 fluxes across an elevation gradient of tropical montane forests. We measured soil CH4 fluxes in a nutrient application experiment at different elevations over a period of 5 years. Nutrient additions increased soil CH4 uptake after 4–5 years of treatment but effects were not uniform across elevations. At 1,000 m, where total soil P was high, we detected mainly N limitation of soil CH4 uptake. At 2,000 m, where total soil P was low, a strong P limitation of soil CH4 uptake was observed. At 3,000 m, where total P was low in the organic layer but high in mineral soil, we found N limitation of soil CH4 uptake. Our results show that projected increases of N and P depositions may increase soil CH4 uptake in tropical montane forests but the direction, magnitude, and timing of the effects will depend on forests' nutrient status and plant‐microbial competition for N and P., Plain Language Summary: CH4 is a potent greenhouse gas that contributes to global warming. Tropical forests are a natural sink of CH4 but increasing nutrient depositions due to industrialization may alter the sink strength of tropical forests. Our results show that projected increases of nitrogen and phosphorus depositions may increase soil CH4 uptake in tropical montane forests but the direction, magnitude, and timing of the effects will depend on forests' nutrients and plant‐microbial competition., Key Points: Projected increases in nitrogen and phosphorus depositions in the tropics will stimulate soil methane uptake in tropical montane forests. The direction, magnitude, and timing of nutrient deposition effects on soil methane uptake will depend on forests' nutrient status. Nutrient limitations on ecosystem processes have to be investigated in actual field conditions., Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659, https://doi.org/10.25625/XLNKNK

Monoculture croplands are commonly associated with deleterious environmental effects due to high fertilization rates. Agroforestry (alternate alleys of trees and crops or alley cropping) has the potential to mitigate the negative environmental effects from agriculture. Understanding the soil-N cycling aids in assessing how the soil function of nutrient cycling is impacted when monoculture system is converted into agroforestry. Currently, there is no systematic comparison in soil-N cycling rates between monoculture and agroforestry croplands in Western Europe. Our study aimed to investigate gross rates of soil-N cycling between agroforestry and monoculture croplands. We measured gross rates of soil-N cycling, using 15N isotopic pool dilution in May-June 2017, at three sites in Germany (Wendhausen, Dornburg, and Forst with Vertic Cambisol, Calcaric Phaeozem, Gleyic Cambisol soils, respectively); each site has paired monoculture and agroforestry systems (established in 2008, 2007, and 2010 at the respective sites). In each management system at each site, we had four replicate plots; for agroforestry system, we conducted measurements in the tree row and within the crop row at 1 m, 4 m, and 7 m from the tree row. The crop management practices in agroforestry crop row and monoculture were the same at each site.For gross rates of ammonium cycling, differences were observed between agroforestry tree row, crop row and monoculture at the site with Vertic Cambisol soil. Higher gross N mineralization rates were observed in monoculture than agroforestry tree row whilst agroforestry tree row exhibited higher gross NH4+ immobilization rates than agroforestry crop row (P < 0.02). This was correlated to higher soil C/N ratio and higher water-filled pore space in the tree row. Tree rows also tend to have higher microbial biomass at all sites. Gross rates of nitrate cycling were higher in the tree row than in the crop row and monoculture at the site with Calcaric Phaeozem soil. This showed a similar pattern with the gene abundance of ammonium oxidizing archeae (AOA), supporting a niche differentiation of AOA by utilizing ammonium mineralized from soil organic matter rather than from fertilizer source. At the site with Vertic Cambisol soil, dissimilatory nitrate reduction to ammonium was very high in the tree row. These changes in soil-N cycling and AOA gene abundance in the tree rows suggest that trees in sites with older agroforestry systems had enhanced the cycling of N in the soil.

Monoculture croplands are considered as major sources of the greenhouse gas, nitrous oxide (N2O). The conversion of monoculture croplands to agroforestry systems, e.g., integrating trees within croplands, is an essential climate-smart management system through extra C sequestration and can potentially mitigate N2O emissions. So far, no study has systematically compared gross rates of N2O emission and uptake between cropland agroforestry and monoculture. In this study, we used an in-situ 15N2O pool dilution technique to simultaneously measure gross N2O emission and uptake over two consecutive growing seasons (2018 - 2019) at three sites in Germany: two sites were on Phaeozem and Cambisol soils with each site having a pair of cropland agroforestry and monoculture systems, and an additional site with only monoculture on an Arenosol soil prone to high nitrate leaching. Our results showed that cropland agroforestry had lower gross N2O emissions and higher gross N2O uptake than in monoculture at the site with Phaeozem soil (P ≤ 0.018 – 0.025) and did not differ in gross N2O emissions and uptake with cropland monoculture at the site with Cambisol soil (P ≥ 0.36). Gross N2O emissions were positively correlated with soil mineral N and heterotrophic respiration which, in turn, were correlated with soil temperature, and with water-filled pore space (WFPS) (r = 0.24 ‒ 0.54, P < 0.01). Gross N2O emissions were also negatively correlated with nosZ clade I gene abundance (involved in N2O-to-N2 reduction, r = -0.20, P < 0.05). These findings showed that across sites and management systems changes in gross N2O emissions were driven by changes in substrate availability and aeration condition (i.e., soil mineral N, C availability, and WFPS), which also influenced denitrification gene abundance. The strong regression values between gross N2O emissions and net N2O emissions (R2 ≥ 0.96, P < 0.001) indicated that gross N2O emissions largely drove net soil N2O emissions. Across sites and management systems, annual soil gross N2O emissions and uptake were controlled by clay contents which, in turn, correlated with indices of soil fertility (i.e., effective cation exchange capacity, total N, and C/N ratio) (Spearman rank’s rho = -0.76 – 0.86, P ≤ 0.05). The lower gross N2O emissions from the agroforestry tree rows at two sites indicated the potential of agroforestry in reducing soil N2O emissions, supporting the need for temperate cropland agroforestry to be considered in greenhouse gas mitigation policies.

The Philippines' agriculture sector continues contributing to country-wide soil trace gas emissions; however, field-based estimation of emissions from this sector remains inadequate. While there’s a need to increase crop production to provide for a fast-increasing population, it is equally essential to reduce greenhouse gases (GHG) from agricultural production to protect the environment. A reliable assessment of soil GHG is a requisite to attain these, thus the present study. We conducted spatially replicated quantification of soil greenhouse gas fluxes, i. e. N2O, CH4, and CO2, with monthly measurements from May 2018 to May 2019, as well astheirsoil controlling factors in nine plots of secondary forest and ten farms, all on comparable Andosol soil in Leyte Island, the Philippines. The management practices of these vegetable farms were those implemented by the farmers, and our measurements were carried out on these actual farm practices, reflecting their commonly varied fertilization rates (200 – 610 kg N ha-1 cropping period-1 with 2 – 3 cropping periods each year). Soil N2O emissions from vegetable farms were larger than the forest (P ≤0.01) and were stimulated during the dry than the wet season (P ≤0.01). Its temporal variation was mainly driven by soil NO3– (r = 0.52, P ≤0.01). Large stocks of soil extractable NO3– in the top 50 cm (9250 ± 2830 mg N m–2, mean ± SE) and 50 – 100 cm soil depth (11255 ± 5980 mg N m–2) supported the substantial soil N2O emissions from these vegetable farms, which were larger than those from other agricultural areas in South East Asia on similar soil and climate. These small-scale vegetable farms had annual fluxes of 12.7 ± 2.6 kg N2O–N, –1.1 ± 0.1 kg CH4–C and 11.6 ± 0.7 Mg CO2–C ha-1 yr-1, whereas the secondary forest as reference land use had 0.10 ± 0.02 kg N2O–N, –2.0 ± 0.2 kg CH4–C, and 8.2 ± 0.7 Mg CO2–C ha-1 yr-1. The forest had larger soil CH4 uptake than the vegetable farms (P ≤0.01). For the forest, CH4 uptake was positively correlated with soil moisture (r = 0.60, P ≤0.01), suggesting diffusion limitation of atmospheric CH4 into the soil and/or soil CH4 production during high rainfall months. For the vegetable farms, CH4 uptake was negatively correlated with both soil NO3– and NH4+ (r = –0.46, P ≤0.01), suggesting CH4 consumption enhanced by mineral N. Soil CO2 emissions were higher in vegetable farms relative to the forest (P ≤0.01), reflecting the former’s reduced soil organic carbon stocks in the top 1 m (P ≤0.01) but compensated with regular application of chicken manure (460 – 1940 kg C ha-1 cropping period-1) as organic fertilizer. Our study expounded understanding on the extent of changes in soil GHG from an Andosol soil that underwent land-use conversion. Such data will be beneficial in developing sound management and policy strategies for reducing soil GHG fluxes from this economically vulnerable agricultural sector.

Monoculture cropland is a major contributor to agriculture-related sources of N2O emission, a potent greenhouse gas and an agent of ozone depletion. Cropland agroforestry has the potential to minimize deleterious environmental impacts. Presently, there is no systematic comparison of soil N2O emission between cropland agroforestry (CAF) and monoculture systems (MC) in Western Europe. Our study aimed to (1) quantify the spatial-temporal dynamics of soil N2O fluxes, and (2) determine their soil controlling factors in CAF and MC. We selected three sites with different soil types (Phaeozem, Cambisol, and Arenosol) in Germany. Each site has paired CAF and MC (agroforestry sites consisted of 12-m wide tree row and 48-m wide crop row and were established in 2007, 2008 and 2019 in these soil types, respectively). In each management system at each site, we had four replicate plots. In the CAF, we conducted measurements in the tree row and within the crop row at 1 m, 7 m, and 24 m from the tree row. We measured soil N2O fluxes monthly over 2 years (March 2018‒February 2020) using static vented chambers method. Following gas sampling, we also measured soil temperature, water-filled pore space (WFPS), and mineral N (NH4+ and NO3-) within the same day. Across all sites, soil moisture and N availability were major drivers of soil N2O fluxes. Both CAF and MC were net sources of soil N2O at all sites. At the site with Phaeozem soil, annual soil N2O emissions from CAF in both years (1.84 ± 0.35 and 1.17 ± 0.30 kg N ha−1 yr−1) were greater than MC (0.89 ± 0.09 and 0.34 ± 0.05 kg N ha−1 yr−1) (P = 0.03). At the site with Cambisol soil, annual soil N2O emission did not differ between MC (0.49 ± 0.07 kg N ha−1 yr−1) and CAF (0.73 ± 0.13 kg N ha−1 yr−1) in 2018/2019 (P = 0.20) whereas in 2019/2020 MC was 134% greater than CAF (2.92 ± 0.45 and 1.25 ± 0.08 kg N ha−1 yr−1, respectively; P = 0.03). The inter-annual differences were largely related to crop types and to climate conditions. At the site with Arenosol soil, there was no difference between CAF and MC. Our results indicated that CAF may decrease, maintain and/or increase soil N2O emissions compared to MC depending on tree age, soil characteristics, management and precipitation.

Rainforest conversion to woody croplands impacts the carbon cycle via ecophysiological processes such as photosynthesis and autotrophic respiration. Changes in the carbon cycle associated with land-use change can be estimated through Land Surface Models (LSMs). The accuracy of carbon flux estimation in carbon fluxes associated with land-use change has been attributed to uncertainties in the model parameters affecting photosynthetic activity, which is a function of both carboxylation capacity (Vcmax) and electron transport capacity (Jmax). In order to reduce such uncertainties for common tropical woody crops and trees, in this study we measured Vcmax25 (Vcmax standardized to 25 °C), Jmax25 (Jmax standardized to 25 °C) and light-saturated photosynthetic capacity (Amax) of Elaeis guineensis Jacq. (oil palm), Hevea brasiliensis (rubber tree), and two native tree species, Eusideroxylon zwageri and Alstonia scholaris, in a converted landscape in Jambi province (Sumatra, Indonesia) at smallholder plantations. We considered three plantations, a monoculture rubber, a monoculture oil palm, and an agroforestry system (jungle rubber plantation), where rubber trees coexist with some native trees. We performed measurements on leaves at the lower part of the canopy, and used a scaling method based on exponential function to scale up photosynthetic capacity related traits to the top of the canopy. At the lower part of the canopy, we found (i) high Vcmax25 values for H. brasiliensis from monoculture rubber plantation and jungle rubber plantation that was linked to a high area-based leaf nitrogen content, and (ii) low value of Amax for E. guineensis from oil palm plantation that was due to a low value of Vcmax25 and a high value of dark respiration. At the top of the canopy, Amax varied much more than Vcmax25 among different land-use types. We found that photosynthetic capacity declined fastest from the top to the lower part of the canopy in oil palm plantations. We demonstrate that photosynthetic capacity related traits measured at the lower part of the canopy can be successfully scaled up to the top of the canopy. We thus provide helpful new data that can be used to constrain LSMs that simulate land-use change related to rubber and oil palm expansion.

Nutrient leaching in intensively managed oil palm plantations can diminish soil fertility and water quality. There is a need to reduce this environmental footprint without sacrificing yield. We quantified nutrient leaching in a large-scale oil palm plantation on Acrisol soil with factorial treatment combinations of two fertilization rates (260 N, 50 P, 220 K kg ha−1 yr−1 as conventional practice, and 136 N, 17 P, 187 K kg ha−1 yr−1, equal to harvest export, as reduced management) and two weeding methods (conventional herbicide, and mechanical weeding as reduced management). Each of the four treatment combinations was represented by a 2500 m2 plot, replicated in four blocks. In each plot, soil-pore water was collected monthly at 1.5 m depth for one year in three management zones: palm circle, inter-row, and frond-stacked area. In the palm circle, nutrient leaching was low due to low solute concentrations and small drainage fluxes, resulting from large plant uptake. Conversely, in the inter-row, nitrate and aluminum leaching losses were high due to their high concentrations, large drainage fluxes, low plant uptake, and acidic pH. In the frond-stacked area, base cation leaching was high, presumably from frond litter decomposition, but N leaching was low. Mechanical weeding, even with conventional high fertilization rates, reduced leaching losses of all nutrients. Mechanical weeding with reduced fertilization had the lowest N and base cation leaching whereas its yield and economic gross margin remain comparable with the conventional management practices. Herbicide weed control decreased ground vegetation, and thereby reduced efficiency of soil nutrient retention. Our findings signified that mechanical weeding and reduced fertilization should be included in the Indonesian Ministry of Agriculture program for precision farming (e.g. variable rates with plantation age), particularly for large-scale plantations, and in the science-based policy recommendations, such as those endorsed by the Roundtable for Sustainable Palm Oil association.

Due to an increasing global demand in cheap oils and biofuels, forest conversion to oil palm plantations is rapidly increasing in Indonesia. Oil palm canopy soil, or the soil lodged between the stems and leaf axils of oil palms, is one ecosystem compartment that has yet to be investigated as to its importance on the soil greenhouse gas budget. Our objectives were 1) to quantify nitrous oxide (N2O) and methane (CH4) fluxes from oil palm canopy soil, and 2) to determine the factors controlling these fluxes. Our study was conducted in Jambi Province, Indonesia, an area of rapid oil palm expansion. Trace gas fluxes were measured in eight smallholder oil palm plantations (9–16 years old). In each plantation, oil palms were delineated into three 1-m height sections (low, middle, and top) to represent possible gradients of canopy soil conditions that influence N2O and CH4 fluxes. Trace gas fluxes were measured by collecting canopy soil from each stem section and immediately incubating in the field in an air-tight glass jar. Canopy soils from all oil palm stem sections emitted N2O and CH4. The top stem section had higher N2O and CH4 emissions than the lower sections, and this pattern was paralleled by nitrogen availability and water content, which strongly influenced these fluxes. Greenhouse gas emissions per unit dry mass of canopy soil were considerable, but on a hectare basis these emissions were small due to the low amount of canopy soil per hectare (170 kg ha−1). Annual canopy soil N2O and CH4 emissions were 10.7 ± 3.3 g N2O-N ha−1 yr−1 and 1.9 ± 0.5 g CH4-C ha−1 yr−1, respectively, which contributed only 1% of the total soil (canopy soil + ground soil) N2O fluxes and 0.2% of the ground soil net CH4 consumption. Our estimate of oil palm canopy soil emissions in Jambi Province were 7.7 Mg N2O-N yr−1 and 1.3 Mg CH4-C yr−1. Considering the increasing areal coverage of oil palm plantations in Southeast Asia, these fluxes may substantially contribute to soil greenhouse gas budgets.

The study aimed to assess soil biochemical properties and nutrient leaching in palm oil plantation. The research was conducted in smallholder oil palm plantations which were located in Jambi Province - Indonesia. Nutrient leaching was determined by measuring nutrient concentration in soil solution bi-weekly and monthly in the frond stacked and fertilized areas; soil water samples were collected by using suction cup lysimeter. The result showed that the application of mineral fertilizer (e.g. NPK) and dolomite resulted higher base saturation, exchangeable Ca, and available P in the fertilized than frond stacked and inter row areas (p ≤ 0.05). Stacking palm oil frond increased the soil macro-porosity, hence decreased leaching of K, Mg, Na, P, and total Al in the frond stacked than in the fertilized areas. The lower leaching losses and the higher soil macroporosity in the frond stacked than in the fertilized areas indicated that either the water did not dilute nutrient in the soil due to bypass flow, or the nutrient release from mineralization did not surpass nutrient demand which is quickly uptaken by palm root. Proper soil management through synchronizing rate of fertilizer application with nutrient output or frequency of fertilizer application may potentially minimize leaching losses.

Peat fires in Indonesia are predominantly anthropogenic and intensified by El Nino Southern Oscillation (ENSO)-related droughts. In recent decades, peat fires, which typically occurred in degraded areas (partially/entirely deforested peatlands), released massive amounts of carbon by burning the peat swamp vegetation and peat substrate. However, it could also influence atmospheric nutrient deposition, have a fertilizing effect on peatlands, and enhance their productivity. Little is known about how ENSO-related peat fires would affect the vegetation in times of little or no human impact. It is also unclear whether in the past such an ash-fertilization effect also occurred in the long term and how fires affected peatland carbon sequestration, which is one of their important ecosystem services. In order to understand this, we examine the fire regime, vegetation composition, nutrient status and carbon (C) accumulation rates of the Air Hitam and Sungai Buluh peatlands in Jambi, Sumatra, Indonesia during the Holocene. The results suggest that increased ENSO frequency and magnitude in the Late Holocene increased the fire occurrence and severity in both sites. During times of little human influence, those fires had little effect on peatland vegetation composition. The coincidence of more frequent fires, a nutrient enrichment and an increased carbon accumulation rate in the Late Holocene suggest that ash-fertilization might have also played a role in the recent past. Thus, low-severity fires could potentially enhance the peatlands' capacity to accumulate carbon if the water table is sufficiently high.

Although plants are recognized as conduits for soil-produced N2O, little is known about N2O fluxes from mature trees under field conditions as well as their contribution to total (soil + stem) N2O fluxes. Exclusion of tree-mediated N2O may thus underestimate total forest N2O fluxes. In the present study, our aims were to quantify tree-mediated N2O emissions and their seasonal patterns. We investigated in situ stem and soil N2O fluxes from mature alder trees on poorly-drained soil and mature beech and spruce trees on well-drained soils during March–October 2015. Alder, beech and spruce consistently emitted N2O via stems and all displayed clear seasonal patterns. Soil and air temperature, vapor pressure deficit, and soil N2O concentration influenced the temporal variability of stem N2O fluxes. Stem and soil N2O fluxes from the alder stand were higher than beech and spruce stands. Stem N2O fluxes accounted 8.8–9.8% of the total N2O fluxes in the beech and spruce stands but only 1.1% in the alder stand. Our study reveals the importance of field-based measurements across seasons in order to come up with reliable estimates of stem N2O fluxes. N2O estimates from temperate forests that are solely based on soil N2O flux measurements are probably conservative.

Soils are not only a major source but also a potential sink of atmospheric nitrous oxide (N 2 O), a potent greenhouse gas and the most important substance causing stratospheric ozone depletion. Net N 2 O flux at the soil surface is a result of two concurrent processes: gross N 2 O emission and gross N 2 O uptake. Little is known about these processes and their controlling factors because it is only in the last five years that the 15 N 2 O pool dilution method was developed to measure these processes in the field. Here, we used this method to quantify gross N 2 O emission and gross N 2 O uptake in adjacent spruce and beech forests on a Cambisol soil in central Germany. Our results showed that the beech stand had higher soil gross and net N 2 O emissions ( P 2 O emission and gross N 2 O uptake were influenced mainly by soil NO 3 − concentration and soil extractable organic C, respectively. The strong correlation between gross N 2 O emission and net N 2 O fluxes suggests that gross N 2 O emission rather than gross N 2 O uptake drove the net N 2 O flux from the soil. Our study showed the underlying mechanisms and controls of gross N 2 O fluxes, which until now were not quantified in actual field conditions across seasons.

The conversion of tropical rainforest to agricultural systems such as oil palm alters biodiversity across a large range of interacting taxa and trophic levels. Yet, it remains unclear how direct and cascading effects of land-use change simultaneously drive ecological shifts. Combining data from a multi-taxon research initiative in Sumatra, Indonesia, we show that direct and cascading land-use effects alter biomass and species richness of taxa across trophic levels ranging from microorganisms to birds. Tropical land use resulted in increases in biomass and species richness via bottom-up cascading effects, but reductions via direct effects. When considering direct and cascading effects together, land use was found to reduce biomass and species richness, with increasing magnitude at higher trophic levels. Our analyses disentangle the multifaceted effects of land-use change on tropical ecosystems, revealing that biotic interactions on broad taxonomic scales influence the ecological outcome of anthropogenic perturbations to natural ecosystems.

Nutrient availability in tropical forest ecosystems plays a critical role in sustaining forest growth and productivity. Observational evidence for nutrient limitations on net primary productivity (NPP) in the tropics is rare yet crucial for predicting the impacts of human-induced changes on tropical forests, particularly for underrepresented tropical regions in Africa. In an ecosystem-scale nutrient manipulation experiment, we assessed the response of different components of above-ground net primary production (ANPP) to nutrient addition of nitrogen (N), phosphorus (P), potassium (K) and all possible combinations (NP, NK, PK, and NPK) at rates of 125 kg N ha-1yr-1, 50 kg P ha-1 yr-1 and 50 kg K ha-1yr-1.We established 32 (8 treatments × 4 replicates) experimental plots of 40 × 40 m2 each and measured stem growth of over 15,000 trees with diameter at breast height (dbh) ≥ 1 cm as well as litter production and above-ground woody biomass production (AWBP), of a lower-montane tropical forest (1100 m a.s.l.) in northwestern Uganda.After 18 months of nutrient addition, we found that different aspects of ANPP, including litter production and AWBP are controlled by multiple soil nutrients. Specifically, we measured higher total fine-litter production in the N (13.6 ± 1.4 Mg ha-1 yr-1) and K (13.3 ± 1.8 Mg ha-1 yr-1) addition plots than the control (11.1 ± 0.6 Mg ha-1 yr-1) plots. Both reproductive litter (flowers and fruits; 10% of total fine-litter fall) and leaf litter (62% of total fine-litter fall) significantly increased with K addition. In general, fine-litter production in our plots is higher than what has been reported so far for lower-montane tropical forests. Increased AWBP is associated with N addition plots. The response of trees to nutrient addition however, varied with tree sizes. Trees with dbh between 10 – 30 cm increased significantly in AWBP under PK addition. There was no effect of nutrient addition associated with either smaller (1 – 10 cm dbh) or larger trees (dbh > 30 cm). The medium-sized trees which may have experienced resource competition but have now transitioned into the canopy layer (exposed to sunlight) are able to use additional nutrient for active growth. In contrast, bigger trees may allocate extra nutrient for reproduction and leaf-vitality, while smaller trees remain shaded, co-limited by sunlight and therefore unable to utilize increased available nutrients for stem diameter growth. ANPP increased by 39% with N addition and marginally by 23% with K additions relative to the control. In conclusion, our experiment provides evidence of N and potentially K limitation of ANPP in this lower-montane tropical forest, and highlights that, in a highly diverse ecosystem different components of ANPP may be regulated by multiple nutrients.

Physical and chemical weathering processes fulfil a crucial function in the biogeochemistry of terrestrial ecosystems. Rock‐derived weathering products provide essential plant nutrients, and regulate the chemical composition of soil, surface, and groundwater. The rate and extent of chemical weathering are influenced by the combined effects of climate, parent material, topography, and vegetation, and ultimately determine the mineral composition and element ratios of soil material. Understanding the spatial variation of rock‐derived weathering products across heterogeneous landscapes not only relies on knowledge of the environmental controls but also of their interactions.High Andean tropical ecosystems provide a good opportunity to study the association between chemical weathering, local topography and vegetation patterns: the climate, parent material and soil age can be held constant at the landscape scale, while the vegetation and slope morphology can vary greatly from the hilltops to the valley bottoms. In this study, we selected 10 soil toposequences on andesitic flows: 5 under tussock grasses, 3 under cushion forming plants and 2 under native forest. A marginally significant increase in base cation depletion is observed along topographic gradients that can be associated with physical transport of weathered soil particles downslope or subsurface water fluxes. Beyond the hillslope-scale topographic control on chemical weathering extent, we observed highly significant differences in chemical weathering extent between vegetation communities with total mass losses in forest soils being respectively 19% and 22% higher than in grasslands and cushion forming plants. Although biotic factors can play a role in creating the observed patterns in soil development, the vegetation communities can also hint to the existence of hillslope micro-topography and subsurface hydrological patterns that are challenging to map in the field.

Over the past decades, excessive use of fertilizers in cropland monocultures in combination with a decrease in fertilizer use efficiency, have led to an increase in nutrient leaching losses, especially for nitrate. Consequently, ground water pollution is widespread and starting to be recognized and potentially sanctioned by the European Union. Unfertilized tree rows alternating with crop rows (e.g. alley-cropping agroforestry) are hypothesized to act as a safety net by taking up excess nutrients below the crop-rooting zone. Here, we measured leaching losses of nitrogen (N), phosphorus (P) and potassium (K) during two growing seasons in agroforestry systems and adjacent monocultures at three sites in Germany, representing a wide range of soil characteristics. Leaching losses of N, P and K were generally lower under agroforestry tree rows at all sites compared to agroforestry crop rows or crop monocultures. Overall, agroforestry reduced nitrate leaching losses by up to 82% compared to monocultures, but showed comparable losses of P and K. Nutrient leaching losses were high in the agroforestry crop rows close to the tree rows where crop productivity is lowest due to resource competition with trees. An adjusted management, e.g. reduced fertilizer inputs close to the tree rows, may counteract these losses. Our results suggest that agroforestry has the potential to reduce nutrient leaching losses through the trees and the application of fertilizer should be reduced in the agroforestry crop row close to the trees. The reduction in nutrient leaching losses in agroforestry indicates an increase in the soil function of water filtration. In order to achieve large-scale implementation of temperate agroforestry, its environmental benefits need to be financially valued for farmers to adapt this widely applicable land use alternative. The presented project, SIGNAL (Sustainable intensification of agriculture through agroforestry) is part of the German research initiative BonaRes (Soil as a sustainable resource).

Agricultural management is a key force affecting soil processes and functions. Triggered by biophysical constraints as well as rapid structural and technological developments, new management practices are emerging with largely unknown impacts on soil processes and functions. This impedes assessments of the potential of such emerging practices for sustainable intensification, a paradigm coined to address the growing demand for food and nonfood products. In terms of soil management, sustainable intensification means that soil productivity is increased while other soil functions and services, such as carbon storage and habitat for organisms, are simultaneously maintained or even improved. In this paper we provide an overview of research challenges to better understand how emerging soil management practices affect soil processes and functions. We distinguish four categories of soil management practices: spatial arrangements of cropping systems, crops and rotations, mechanical pressures, and inputs into the soil. Key research needs identified for each include nutrient efficiency in agroforestry versus conventional cropping systems, soil-rhizosphere microbiome elucidation to understand the interacting roles of crops and rotations, the effects of soil compaction on soil–plant–atmosphere interactions, and the ecotoxicity of plastics, pharmaceuticals and other pollutants that are introduced into the soil. We establish an interdisciplinary, systemic approach to soil science and include cross-cutting research activities related to process modeling, data management, stakeholder interaction, sustainability assessment and governance. The identification of soil research challenges from the perspective of agricultural management facilitates cooperation between different scientific disciplines in the field of sustainable agricultural production.

Although tree stems act as conduits for greenhouse gases (GHG) produced in the soil, the magnitudes of tree contributions to total (soil + stem) nitrous oxide (N2O) emissions from tropical rainforests on heavily weathered soils remain unknown. Moreover, soil GHG fluxes are largely understudied in African rainforests, and the effects of land-use change on these gases are identified as an important research gap in the global GHG budget. In this study, we quantified the changes in stem and soil N2O fluxes with forest conversion to cacao agroforestry. Stem and soil N2O fluxes were measured monthly for a year (2017–2018) in four replicate plots per land use at three sites across central and southern Cameroon. Tree stems consistently emitted N2O throughout the measurement period, and were positively correlated with soil N2O fluxes. 15N-isotope tracing from soil mineral N to stem-emitted 15N2O as well as correlations between temporal patterns of stem N2O emissions, soil-air N2O concentration, soil N2O emissions, and vapor pressure deficit suggest that N2O emitted by the stems originated predominantly from N2O produced in the soil. Forest conversion to extensively managed, mature (> 20 years old) cacao agroforestry had no effect on stem and soil N2O fluxes. The annual total N2O emissions were 1.55 ± 0.20 kg N ha−1 yr−1 from the forest and 1.15 ± 0.10 kg N ha−1 yr−1 from cacao agroforestry, with tree N2O emissions contributing 1 to 38 % for forests and 8 to 15 % for cacao agroforestry. These substantial contributions of tree stems to total N2O emissions highlight the importance of including tree-mediated fluxes in ecosystem GHG budgets. Taking into account that our study sites’ biophysical characteristics represented two-thirds of the humid rainforests in the Congo Basin, we estimated a total N2O source strength for this region of 0.18 ± 0.05 Tg N2O yr−1.

Agroforestry, which is the integration of trees into monoculture cropland, can alter soil properties and nutrient cycling. Temperate agroforestry practices have been shown to affect soil microbial communities as indicated by changes in enzyme activities, substrate-induced respiration, and microbial biomass. Research exploring soil microbial communities in temperate agroforestry with the help of molecular tools which allow for the quantification of microbial taxa and selected genes is scarce. Here, we quantified 13 taxonomic groups of microorganisms and nine genes involved in N cycling (N2 fixation, nitrification, and denitrification) in soils of three paired temperate agroforestry and conventional monoculture croplands using real-time PCR. The agroforestry croplands were poplar-based alley-cropping systems in which samples were collected in the tree rows as well as within the crop rows at three distances from the tree rows. The abundance of Acidobacteria, Actinobacteria, Alpha- and Gammaproteobacteria, Firmicutes, and Verrucomicrobia increased in the vicinity of poplar trees, which may be accounted for by the presence of persistent poplar roots as well as by the input of tree litter. The strongest population increase was observed for Basidiomycota, which was likely related to high soil moisture, the accumulation of tree litter, and the absence of tillage in the tree rows. Soil microorganisms carrying denitrification genes were more abundant in the tree rows than in the crop rows and monoculture systems, suggesting a greater potential for nitrate removal through denitrification, which may reduce nitrate leaching. Since microbial communities are involved in critical soil processes, we expect that the combination of real-time PCR with soil process measurements will greatly enhance insights into the microbial control of important soil functions in agroforestry systems. Open-Access-Publikationsfonds 2020 peerReviewed