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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.

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.

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).

Land-use change has a strong impact on carbon, energy and water fluxes and its effect is particularly pronounced in tropical regions. Uncertainties exist in the prediction of future land-use change impacts on these fluxes by land surface models due to scarcity of suitable measured data for parametrization and poor representation of key biogeochemical processes associated with tropical vegetation types. Rubber plantations (Havea brasilliensis) are a crucial land-use type across tropical landscapes that has greatly expanded in recent decades. Here, we first synthesize the relevant data for describing the biogeochemical processes of rubber from our past measurement campaigns in Jambi province, Indonesia. We then use these data-sets to develop a rubber plant functional type (PFT) for the Community Land Model (CLM4.5). Field measured data from small-holder plantations on leaf litterfall, soil respiration, latex harvest, leaf area index, transpiration, net primary productivity, and above-ground and fine root biomass were used to develop and calibrate a new PFT-based model (CLM4.5-rubber). CLM-rubber predictions adequately captured the annual net primary productivity and above-ground biomass as well as the seasonal dynamics of leaf litterfall, soil respiration, soil moisture and leaf area index. All of the predicted water fluxes of CLM-rubber were very similar to a site-specific calibrated soil water model. Including temporal variations in leaf life span enabled CLM-rubber to better capture the seasonality of leaf litterfall. Increased sensitivity of stomata to soil water stress and the enhancement of growth and maintenance respiration of fine roots in response to soil nutrient limitation enabled CLM-rubber to capture the magnitude of transpiration and leaf area index. Since CLM-rubber predicted reasonably well the carbon and water use, we think that the current model can be used for larger-scale simulations within Jambi province because more than 99 % of the rubber plantations are smallholder owned in Jambi province and have low soil fertility.

Conversion of forest to rubber and oil palm plantations is widespread in Sumatra, Indonesia, and it is largely unknown how such land-use conversion affects nutrient leaching losses. Our study aimed to quantify nutrient leaching and nutrient retention efficiency in the soil after land-use conversion to smallholder rubber and oil palm plantations. In Jambi province, Indonesia, we selected two landscapes on highly weathered Acrisol soils that mainly differed in texture: loam and clay. Within each soil type, we compared two reference land uses, lowland forest and jungle rubber (defined as rubber trees interspersed in secondary forest), with two converted land uses: smallholder rubber and oil palm plantations. Within each soil type, the first three land uses were represented by 4 replicate sites and the oil palm by three sites, totaling 30 sites. We measured leaching losses using suction cup lysimeters sampled biweekly to monthly from February to December 2013. Forests and jungle rubber had low solute concentrations in drainage water, suggesting low internal inputs of rock-derived nutrients and efficient internal cycling of nutrients. These reference land uses on the clay Acrisol soils had lower leaching of dissolved N and base cations (P= 0.01–0.06) and higher N and base cation retention efficiency (P P P P=0.08) and organic C (P&thinsp

Abstract. Presently, the lack of data on soil organic carbon (SOC) in relation to land-use types and biophysical characteristics prevents reliable estimates of carbon stocks in montane landscapes of mainland SE Asia. Our study, conducted in a 10,000-hectare landscape in Xishuangbanna, SW China, aimed at assessing the spatial variability in SOC and its relationships with land-use cover and key biophysical characteristics at multiple spatial scales. We sampled 27 one-hectare plots including 10 plots in mature forests, 11 plots in regenerating or highly disturbed forests, and six plots in open land including tea plantations or grasslands. We used a sampling design with a hierarchical structure. The landscape was first classified according to land-use types. Within each land-use type, sampling plots of 100 m × 100 m each were randomly selected, and within each plot we sampled nine subplots. This hierarchical sampling design allowed partitioning of the overall variance in SOC, vegetation, soil properties and topography that was accounted for by the variability among land-use types, among plots nested within land-use types, and within plots. SOC concentrations and stocks did not differ significantly across land-use types. The SOC stocks to a depth of 0.9 m were 177.6 &pm; 19.6 Mg C ha−1 in tea plantations, 199.5 &pm; 14.8 Mg C ha−1 in regenerating or highly disturbed forests, 228.6 &pm; 19.7 (SE) Mg C ha−1 in mature forests, and 236.2 &pm; 13.7 Mg C ha−1 in grasslands. In this montane landscape, variability within plots accounted for more than 50 % of the overall variance in SOC. The relationships between SOC, biophysical characteristics and land-use types varied across spatial scales. Variability in SOC within plots was determined by tree basal area, litter layer carbon stocks and slope. Variability in SOC among plots in open land was influenced by land-use type – SOC concentrations and stocks in grasslands were higher than in tea plantations. In forests, the variability in SOC among plots was related to elevation. The scale-dependent relationships between SOC and its controlling factors demonstrate that studies which aim to investigate the land-use effects on SOC need an appropriate sampling design reflecting the controlling factors of SOC so that land-use effects will not be masked by the variability between and within sampling plots.

Oil palm and rubber plantations cover large areas of former rainforest in Sumatra, Indonesia, supplying the global demand for these crops. Although forest conversion is known to influence soil nitrous oxide (N2O) and nitric oxide (NO) fluxes, measurements from oil palm and rubber plantations are scarce (for N2O) or nonexistent (for NO). Our study aimed to (1) quantify changes in soil-atmosphere fluxes of N oxides with forest conversion to rubber and oil palm plantations, and (2) determine their controlling factors. In Jambi, Sumatra, we selected two landscapes that mainly differed in texture but both on heavily weathered soils: loam and clay Acrisol soils. Within each landscape, we investigated lowland forest, rubber trees interspersed in secondary forest (termed as jungle rubber), both as reference land uses, and smallholder rubber and oil palm plantations, as converted land uses. Each land use had four replicate plots within each landscape. Soil N2O fluxes were measured monthly from December 2012 to December 2013, and soil NO fluxes were measured four times between March and September 2013. In the loam Acrisol landscape, we also conducted weekly to bi-weekly soil N2O flux measurements from July 2014 to July 2015 in a large-scale oil palm plantation with four replicate plots for comparison with smallholder oil palm plantations. Land-use conversion to smallholder plantations had no effect on soil N-oxide fluxes (P = 0.58 to 0.76) due to the generally low soil N availability in the reference land uses that further decreased with land-use conversion. Over one-year measurements, the temporal patterns of soil N-oxide fluxes were influenced by soil mineral N and water contents. Across landscapes, annual soil N2O emissions were controlled by gross nitrification and sand content, which also suggest the influence of soil N and water availability. Soil N2O fluxes (µg N m−2 h−1) were: 7 ± 2 to 14 ± 7 (reference land uses), 6 ± 3 to 9 ± 2 (rubber), 12 ± 3 to 12 ± 6 (smallholder oil palm), and 42 ± 24 (large-scale oil palm). Soil NO fluxes (µg N m−2 h−1) were: −0.6 ± 0.7 to 5.7 ± 5.8 (reference land uses), −1.2 ± 0.5 to −1.0 ± 0.2 (rubber) and −0.2 ± 1.2 to 0.7 ± 0.7 (smallholder oil palm). The low N fertilizer application in smallholder oil palm plantations (commonly 48 to 88 kg N ha−1 yr−1) resulted in N-oxide losses of only 0.2–0.7 % of the applied N. To improve estimate of soil N-oxide fluxes from oil palm plantations in this region, studies should focus on large-scale plantations (which usually have two to four times higher N fertilization rates than smallholders) with frequent measurements following fertilizer application.

Soil respiration is the second largest flux in the global carbon cycle, yet the underlying below-ground process, carbon dioxide (CO2) production, is not well understood because it can not be measured in the field. CO2 production has frequently been calculated from the vertical CO2 diffusive flux divergence, known as "soil-CO2 profile method". This relatively simple model requires knowledge of soil CO2 concentration profiles and soil diffusive properties. Application of the method for a tropical lowland forest soil in Panama gave inconsistent results when using diffusion coefficients (D) calculated based on relationships with soil porosity and moisture ("physically modeled" D). Our objective was to investigate whether these inconsistencies were related to (1) the applied interpolation and solution methods and/or (2) uncertainties in the physically modeled profile of D. First, we show that the calculated CO2 production strongly depends on the function used to interpolate between measured CO2 concentrations. Secondly, using an inverse analysis of the soil-CO2 profile method, we deduce which D would be required to explain the observed CO2 concentrations, assuming the model perception is valid. In the top soil, this inversely modeled D closely resembled the physically modeled D. In the deep soil, however, the inversely modeled D increased sharply while the physically modeled D did not. When imposing a constraint during the fit parameter optimization, a solution could be found where this deviation between the physically and inversely modeled D disappeared. A radon (Rn) mass balance model, in which diffusion was calculated based on the physically modeled or constrained inversely modeled D, simulated observed Rn profiles reasonably well. However, the CO2 concentrations which corresponded to the constrained inversely modeled D were too small compared to the measurements. We suggest that, in well-structured soils, a missing description of steady state CO2 exchange fluxes across water-filled pores causes the soil-CO2 profile method to fail. These fluxes are driven by the different diffusivities in inter- vs. intra-aggregate pores which create permanent CO2 gradients if separated by a "diffusive water barrier". These results corroborate other studies which have shown that the theory to treat gas diffusion as homogeneous process, a precondition for use of the soil-CO2 profile method, is inaccurate for pore networks which exhibit spatial separation between CO2 production and diffusion out of the soil.