Effect of long-term tillage and cropping system on portion of fungal and bacterial necromass carbon in soil organic carbon

The microbial necromass carbon (M-C) is considered to be relatively stable and important part of SOC. However, knowledge about the contribution of M-C storage in increasing SOC storage and the influences of tillage and cropping systems on fungal and bacterial necromass carbon (F-C and B-C) is lacking, especially in the long-term. Here, a subset of treatments in a long-term 16-year study was used to evaluate these changes. The treatments selected for present study were: (1) NTCS: no-tillage with two year corn-soybean (CS) rotation (Zea mays L. – Glycine max Merr.); (2) MPCS: moldboard plowing with two year corn-soybean rotation; (3) NTCC: no-tillage with continuous corn (CC); (4) MPCC: moldboard plowing with continuous corn; (5) CTCC: conventional tillage with continuous corn and no residue return (traditional tillage practice in China). Amino sugars were measured to calculate the M-C, F-C and B-C and their storage. Three-way ANOVA showed that tillage, depth and their interactions had significant effect on all amino sugars. However, two-way ANOVA in separate layers showed the effects of tillage on all amino sugars mostly occurred in 0–5 cm layer. A decaying exponential model showed the relationship between the M-C and SOC (R² = 0.87). Tillage showed great effects on the amount of M-C, F-C and B-C storage but had no influence on the proportion of their distribution in SOC storage (%). More than half of the increase in SOC storage existed as M-C storage under CC cropping due to returned residue, which was higher than CS. F-C storage % was not affected by agriculture management (residue return, tillage and cropping), whereas B-C storage % was affected by both quantity and quality of residue. The results suggested that CC cropping system was better for M-C sequestration and that bacteria were relatively more sensitive to agriculture management than fungi.