Effects of Maize Shoot and Root Residues Added on Microbial Residue Carbon and Nitrogen in Different Fertility Levels of Black Soil

Maize shoot and root residues are the significance sources of soil organic matter (SOM). Exogenous carbon (C) and nitrogen (N) are assimilated by soil microorganism, accumulated as microbial necromass and contributed to relatively stable SOM. The objectives of this study were to clarify the accumulation of microbial residue C and N in different fertility levels of soil with shoot and root residues addition, and to qualify the contributions of microbial residue C and N to soil organic C (SOC) and total N (TN), respectively, so as to provide a basis for increasing the storage and stability of SOC and TN.

Based on a long-term experiment of black soil with different fertility levels, this study used the ¹³C¹⁵N isotope labeling and amino sugar biomarkers methods, and mixed the labeled maize shoot and root residues with black soil samples. Then the mixture samples were incubated in lab, and sampled in 30 and 180 days. The percentages of exogenous C (shoot C and root C) and N (shoot N and root N) remaining in soil, contents of microbial residues C and N, and their contributions to SOC and TN were analyzed.

On the 180th day, the percentages of shoot C and root C remaining in soil were, on average, 36.3% and 31.7%, respectively, and those of shoot N and root N remaining in soil were, on average, 95.8% and 79.3%, respectively. On the 180th day, the ratio of the SOC derived from exogenous C (¹³C-SOC) to TN derived from exogenous N (¹⁵N-TN) were, on average, 17.6 and 28.5 in the soils with shoot and root residue addition, respectively. The ratio of ¹³C-SOC to ¹⁵N-TN was decreased by 47.9% and 28.2% in the soils with shoot and root residue addition on the 180th day compared with the 30th day, respectively. During the incubation, the contents of fungal and bacterial residue in the high fertility soil were, on average, 1.17 and 1.31 times those in the low fertility soil, respectively. On the 180th day, the content of microbial residue C and N (fungal and bacterial) was, on average, 8.5% higher in the soil added with shoot than that in the soil added with root. On the end of incubation, the average percentage of fungal residue C contributed to SOC was 37.0% and 33.8%, respectively, and that of bacterial residue C contributed to SOC was 11.2% and 9.2% in the high and low fertility level soils, respectively. The average percentage of fungal residue C contributed to SOC was 36.0% and 34.7%, and that of bacterial residual C contributed to SOC was 10.8% and 9.6% on the end of incubation, respectively. The average percentage of bacterial residue C contributed to SOC was 55.2% and 16.3% in the soil added with shoot and root on the 30th day, respectively. The average percentage of fungal residue N contributed to TN was 63.5% and 60.5%, and that of bacterial residue N to TN was 16.4% and 17.5% in low and high fertility level soils on the 180th day, respectively. The percentage of bacterial residue C and N contributed to SOC and TN was increased by 4.8% and 7.4% in the high fertility soil and by 20.3% and 32.5% in low fertility soil on the 180th day compared with the basic soil without shoot and root addition, respectively.

The accumulation of fungal residue was of great significance for the storage and stability of SOC and TN. Maize shoot addition promoted the accumulation of microbial residues C and N in soil compared with root addition. Low fertility soil with the addition of shoot and root improved the transformation of bacterial residue C and N to SOC and TN.