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In the Vietnamese Mekong Delta, soil quality and crop yield are steadily declining under rice monocultures with three crops per year. The objective of this study was to evaluate the medium-term effects of rotating rice with upland crops and adding organic amendments on rice yield, and to relate this to soil quality. A field trial with split-plot design including two factors and three replicates was carried out from 2017 to 2020, over the course of nine consecutive cropping seasons. Crop rotations and organic amendments were applied as main-plot and subplot factors, respectively. The rotations were (1) rice–rice–rice (R–R–R), (2) soybean–rice–rice (So–R–R), and (3) sesame–rice–rice (Se–R–R), while organic amendment treatments included (i) no amendment (NO-AM), (ii) compost of rice straw and cow manure (RS+CM), and (iii) sugarcane compost (SGC); the composts were applied at a rate of 2.0 t ha−1. The rotation cycle started with the so-called spring–summer (SS) season, followed by the summer–autumn (SA) season and ending with the winter–spring (WS) season. Rice yield significantly (p < 0.05) increased under organic amendments after nine growing seasons (2019–2020 WS), with an increment of 5.1% for RS+CM (7.07 ton/ha) and 6.1% for SGC (7.14 ton/ha). Contrary to our expectation, rotations with upland crops did not significantly increase rice yield. Rice yield significantly and positively correlated with an integrated soil quality index–SQI (r = 0.85) for the topsoil (0–15 cm), but not for the subsoil (15–30 cm). The increased availability of soil nutrients (Si and marginally also P) and improved soil physical properties probably induced by organic amendments, along with other soil properties under study, cumulatively attributed to enhanced rice yield. Repeated organic amendments thus becomes an effective management practice in improving soil quality under rice-based systems and could be applied to sustain rice yield in rice-producing regions with similar soil types and climatic conditions. Use of a SQI involving several soil quality indicators enables us to quantify the overall importance of soil fertility for rice yield versus other factors, and it provides an effective means of quantifying the integrated effect of improved management. Moreover, integrating a wide range of soil quality indicators in a SQI ensures its applicability across diverse settings, including different crop rotations and various soil types. [ABSTRACT FROM AUTHOR]

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 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]

Termites can play a localized prominent role in soil nutrient availability and cycling because mound materials are often enriched in nutrients relative to surrounding soil. Mound materials may thus prove to be useful amendments, though evidently mound spatial arrangement needs to be considered as well. Furthermore, it is not known if gradients of soil properties exist from termite mound to interspace sites. Studying both aspects would be required to decide whether spreading of mounds or spatially differentiated management of surrounding crop to accommodate soil fertility gradients would be valid nutrient-management strategies. Mound abundance and mass were estimated at 9 and 4 mounds ha−1, representing 38.9 and 6.3 t ha−1 on Nitisols and Vertisols, respectively. Soil physical and chemical properties were measured on samples collected from internal and external parts of mounds and adjacent soils at 0.5, 1 and 10 m away from mounds. In general, termite mounds were enriched in plant nutrients and SOC on Vertisols but not on Nitisols. Termite mounds constituted only 0.3 to 1.3% of the 0–15 cm SOM stock on a per ha basis but nevertheless the immediate vicinity of termite mounds was a relative fertile hotspot. Hence, under the studied condition, we suggest spatial arrangement of crop around termite mounds according to soil fertility gradient and spatially differentiated nutrient management strategies. Our result suggests recommendation of termite mound spreading for soil nutrient amendment has to consider plant nutrient stock in termite mounds on per ha basis besides their nutrient enrichment. Interesting topics for future investigation would be growth experiment for different crops with mound materials treatment. It would also be interesting to study the effect mound building termite on soil properties under different soil conditions, slope class and land use. [ABSTRACT FROM AUTHOR]