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Soil type and fertility level influence straw carbon dynamics in the agroecosystems. However, there is a limited understanding of the dynamic processes of straw-derived and soil-derived carbon and the influence of the addition of straw carbon on soil-derived organic carbon in different soils associated with different fertility levels. In this study, we applied the in-situ carborundum tube method and 13C-labeled maize straw (with and without maize straw) at two cropland (Phaeozem and Luvisol soils) experimental sites in northeast China to quantify the dynamics of maize-derived and soil-derived carbon in soils associated with high and low fertility, and to examine how the addition of maize carbon influences soil-derived organic carbon and the interactions of soil type and fertility level with maize-derived and soil-derived carbon. We found that, on average, the contributions of maize-derived carbon to total organic carbon in maize-soil systems during the experimental period were differentiated among low fertility Luvisol (from 62.82% to 42.90), high fertility Luvisol (from 53.15% to 30.00%), low fertility Phaeozem (from 58.69% to 36.29%) and high fertility Phaeozem (from 41.06% to 16.60%). Furthermore, the addition of maize carbon significantly decreased the remaining soil-derived organic carbon in low and high fertility Luvisols and low fertility Phaeozem before two months. However, the increasing differences in soil-derived organic carbon between both soils with and without maize straw after two months suggested that maize-derived carbon was incorporated into soil-derived organic carbon, thereby potentially offsetting the loss of soil-derived organic carbon. These results suggested that Phaeozem and high fertility level soils would fix more maize carbon over time and thus were more beneficial for protecting soil-derived organic carbon from maize carbon decomposition. [ABSTRACT FROM AUTHOR]

Crop residue serves as a vital source of energy and nutrients for microbial proliferation, ultimately contributing to the formation of soil organic carbon (SOC). Labile organic C pool (i.e., extractable organic C and microbial biomass C) not only represents a small part of SOC, but also is a sensitive indicator of soil biogeochemical process under agricultural management. However, the interaction between maize residue types and initial soil C status on the fate of residue C in labile organic C pools remains not very clear. Here, we added the 13C-labeled maize root, stem, and leaf residues into soils with low and high initial C status for 360 days of incubation and analyzed the contents of residue derived- salt-extractable organic C (SEOC) and microbial biomass C (MBC). The results showed that average 7.31 % and 6.46 % of SEOC was derived from maize residue C in the low and high C soil during the whole incubation, respectively. The contribution of MBC derived from residue C to total MBC in the high C soil was on average 13.36 % higher than that in the low C soil, which indicated that the high C soil could accelerate the transformation of residue C into microbial biomass by efficient anabolism pathway. The distribution percentage of leaf residue C to MBC was on average 1.73 % higher than those of root residue C and stem residue C to MBC in the low C soil, whereas more root residue C was entered to SEOC and MBC compared with stem residue C and leaf residue C in the high C soil, which demonstrated that the low-quality residue (i.e., maize root) tended to be more susceptible to microbial utilization, but it depended on the initial soil C status. Overall, these findings contribute to understanding the mechanisms of microbial-mediated C transformation processes, and provide insights into the capture and incorporation of plant residue C into labile organic C pools driven by initial soil C status and crop residue returning. • Root residue combined with high C soil contributed more to SEOC and MBC compared with stem and leaf residues. • Leaf C derived- MBC was higher than root and stem C derived- MBC in the low C soil. • Low C soil promotes the accumulation of residue C as SEOC relative to high C soil. • High C soil promotes the incorporation of residue C into MBC relative to low C soil. [ABSTRACT FROM AUTHOR]

Long-term intensive cultivation leads to the decrease of soil organic carbon (SOC) and soil fertility. Crop residue amendment to soil is documented as an effective measure to increase SOC and improve soil productivity. However, there is limited information on the turnover and storage of crop residue carbon (C) in soil aggregates after the residue is added to soil with low fertility. The objectives of this research were to investigate the distribution and storage of residue C in soil aggregates and its contribution to different physical fractions of SOC, and to quantify the turnover of residue C in soil with low fertility. Soil samples added with 13 C-labelled maize straw residue were put into carborundum tubes for two-year long in-situ incubation. Soil aggregates were separated by wet sieving and then physically fractionated. During the whole incubation process, 12–15% of residue C was stably distributed to 2000–250 μm aggregates, while the percentage of residue C distributed to microaggregates (<250 μm) increased with incubation time. The contribution of residue C to particulate organic C (POC) fractions decreased from average 63% on day 60 to average 43% on day 720 and that to mineral-associated organic C (mSOC) fraction increased from average 23% on day 60 to average 28% on day 720. More than 50% of fine POC (fPOC) was derived from residue C, especially 71% in microaggregates on day 360. Within aggregates, the percentages of residue C distributed to free light organic C (fLOC) and coarse POC (cPOC) reduced and these to fPOC and mSOC strengthened with incubation time. Mean residence time (MRT) of residue C was shortened with the increase of the aggregate sizes. MRT of mSOC was longer compared to other SOC physical fractions. These results suggest that microaggregates could provide favorable conditions for microbial activities and conduce to fPOC accumulation in a low fertility soil amendment with crop residue. [ABSTRACT FROM AUTHOR]