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The untapped potential for carbon sequestration in agricultural soils represents one of the most cost-effective tools for climate change mitigation. Increasing soil organic matter also brings other agronomic benefits such as improved soil structure, enhanced water-and-nutrient-retention capacity, and biological activity. Broadly, soil organic carbon storage is achieved by increasing carbon inputs (plant residues and organic amendments) and reducing carbon outputs (soil loss mechanisms, decomposition). With a focus on carbon inputs—more specifically, organic amendments—as leverage to increase soil organic carbon, we compared the respiration rates and carbon storage of incubated soil cores amended with maize straw, manure, two digestates and the solid fraction of digestate. Using the variation in the natural 13C abundance found in C4 and C3 plants as a tracer, we were able to partition the CO2 emissions between the exogenous organic matter materials elaborated from maize (C4) and native soil organic carbon (C3). The addition of digestate resulted in an additional 65 to 77% of remaining organic carbon after 92 days. The digestate-derived CO2 was fitted to a second-order kinetic carbon model that accounts for the substrate C that is assimilated into the microbial biomass. The model predicted a carbon sequestration potential of 56 to 73% of the total applied organic carbon after one to two years. For the solid fraction, the results were higher, with 89% of the applied organic carbon after 92 days and a sequestration potential of 86%. The soil priming ranged from −19% to +136% in relation to the unamended control soil, highlighting a surprisingly wide spectrum of results that warrants the need for further research on soil–digestate interactions. [ABSTRACT FROM AUTHOR]

The occurrence and spread of invasive woody species are a truly global phenomenon, but tropical regions seem to be particularly vulnerable due to high rates of soil degradation in combination with climate change, and limited resources for containment. There is increasing awareness that complete eradication programs are often not effective. The existence of many "controversial species," i.e., species with both negative and positive impacts, renders decision-making processes for management exceedingly complex. By providing a very extensive overview of the current state of knowledge on impacts and containment strategies of invasive woody species, we aim to help underpin such decisions. We discuss both negative impacts and potential benefits of invasive woody species, focusing on the two most important ones, namely animal fodder production and the positive impacts on soil functioning and soil quality. Invasive woody species can positively impact livestock production (1) indirectly by improving pasture quality because of improved soil quality and functioning, and (2) directly by supplying a high-quality protein component for animal fodder. Invasive woody species increase soil carbon sequestration and nitrogen and phosphorus availability depending on the density of the invader, its capacity to fix nitrogen, the quantity and quality of its litter, and the direct interactions between its roots and the soil microbial community. The balance between potential benefits and risks depends to a large extent on the interaction with the local environment (climate, soil, vegetation, and animals) and the socioeconomic context of each region. When an invasion process starts because there is no local predator, then management can target eradication or very strict containment. If the invasion is the result of strong disturbance of the ecosystem, then intensive but well-thought management of the invasive species would be the choice to be made, as this may help to restore the ecosystem. [ABSTRACT FROM AUTHOR]

Purpose: This study investigated the C and N mineralisation potential of solid fractions (SFs) from co-digestated pig manure after P-stripping (P-POOR SF) in comparison with P-rich SFs, as a means to estimate their organic matter stability in soil. Compost (COMP) and biochar (BCHR) (made from P-POOR SF) were also included in the study as reference biosolids. Methods: The SFs were incubated in a sandy-loam soil under moist conditions to determine production of CO2 and mineral N. At specified intervals, CO2 evolution in the mixtures was measured via the alkali trap method and titration over a period of 81 days, while mineral N was measured using a flow analyser after KCl extraction over a period of 112 days. Results: The various SFs showed similar patterns of C mineralisation (15–26% of added total C in 81 days) that were clearly higher than for COMP and BCHR (6% and 7%, respectively). Temporary N immobilisation was observed in biosolids with a high C/N ratio. The effective organic matter (EOM) of the SFs was calculated based on the C mineralisation data and varied between 130 and 369 kg Mg−1. Conclusions: The SF with a reduced P content had a high EOM/P ratio which is beneficial in areas where P status of the soil is already high. Moreover, the N mineralisation patterns confirm that a high C/N ratio may also reduce risks for N leaching due to temporary N immobilisation. [ABSTRACT FROM AUTHOR]

The carbon (C) and nitrogen (N) mineralisation rates of five digestates were studied and compared with pig slurry, compost, and a solid fraction of digestate in aerobic incubation experiments. The objective was to identify the most relevant drivers of C and N mineralisation based on the physicochemical properties of the products. Net organic nitrogen mineralisation of digestates (Nmin,net) was on average 30%, although the range was relatively wide, with digestate from pig manure (39%) reaching double the value of digestate from sewage sludge (21%). The total carbon to total nitrogen (TC:TN) (r = −0.83, p < 0.05) and ammonium nitrogen to total nitrogen (NH4+-N:TN) (r = 0.83, p < 0.05) ratios of the products were strongly correlated with Nmin,net, adequately mirroring the expected fertilising potential of the products. The digestates had C sequestration values between 50 and 81% of applied total organic carbon (TOC), showcasing their potential to contribute to C build-up in agricultural soils. The carbon use efficiency of the amended soils was negatively correlated with dissolved organic carbon (DOC) (r = −0.75, p < 0.05) suggesting that catabolic activities were promoted proportionately to the DOC present in these products. Ratios of DOC:TOC (r = −0.88, p < 0.01) and TC:TN (r = 0.92, p < 0.01) were reliable predictors of the fraction of C that would remain one year after its incorporation and thus could be used as simple quality parameters to denote the C sequestration potential of digestates prior to their use in the field. [ABSTRACT FROM AUTHOR]

Abstract Evaluations of soil organic carbon (SOC) stocks are often based on assigning a carbon density to each one of a number of ecosystems or soil classes considered, using data from soil profiles within these categories. A better approach, in which the use of classification methods by which extrapolation of SOC data to larger areas is avoided, can only be used if enough data are available at a sufficiently small scale. Over 190 000 SOC measurements (0–24 cm) have been made in the Flemish cropland (the Northern part of Belgium) in the 1989–2000 period. These SOC data were grouped into 3-year periods and as means plus standard deviation per (part of) community (polygons). This large dataset was used to calculate SOC stocks and their evolution with time, without data extrapolation. Using a detailed soil map, larger spatial groups of polygons were created based on soil texture and spatial location. Linear regression analysis showed that in the entire study area, SOC stocks had decreased or at best had remained stable. In total, a yearly decrease of 354 kton OC yr-1 was calculated, which corresponds with a net CO2 emission of 1238 kton CO2 yr-1 . Specific regions with a high carbon sequestration potential were identified, based on SOC losses during the 1989–2000 period and the mean 1999 SOC content, compared to the average SOC content of soils in Flanders with a similar soil texture. When restoring the SOC stocks to their 1990 level, we estimated the carbon sequestration potential of the Flemish cropland soils to be some 300 kton CO2 yr-1 at best, which corresponds to a 40-year restoration period. In conclusion, we can say that in regions where agricultural production is very intense, carbon sequestration in the cropland may make only a very modest contribution to a country's effort to reduce greenhouse gas emissions. [ABSTRACT FROM AUTHOR]