Contribution of soil microbial necromass carbon to soil organic carbon in grassland under precipitation change and its influencing factors in loess hilly region, Northwest China.

To investigate the contribution of microbial necromass carbon (MNC) to soil organic carbon (SOC) and its influencing factors under precipitation changes in grassland, we conducted a precipitation experiment with seven different precipitation levels in the <i>Bothriochloa ischaemum</i> restoration area in the loess hilly region. We analyzed the contents and characteristics of fungal necromass carbon (FNC), bacterial necromass carbon (BNC), and MNC in different fractions of SOC under different treatments, including natural precipitation (CK), and increased and decreased 20%, 40%, 60% of natural precipitation (I₂₀, I₄₀, I₆₀, D₂₀, D₄₀, D₆₀) . The results showed that 1) MNC content in mineral organic carbon (MAOC) ranged from 1.62 g·kg^-1 to 2.17 g·kg^-1, which was higher than that in particulate organic carbon (POC) ranging from 0.69 g·kg^-1 to 1.31 g·kg^-1. The former was approximately 1.4 to 2.8 times as that of the latter. 2) FNC and MNC exhibited similar changes in both MAOC and POC fractions. BNC content in MAOC was approximately 1-3.1 times as that of FNC. FNC content in POC was generally higher than BNC except for I₄₀ and I₆₀ where BNC exceeded FNC. 3) Overall, both increases and decreases in precipitation resulted in elevated MNC/MAOC and BNC/MAOC ratios, but decreased MNC/POC and FNC/POC ratios. The MNC/MAOC ratios in I₆₀ and D₆₀ were 33.2% and 18.1% higher than CK, respectively. The BNC/MAOC ratios in D₆₀, I₄₀ and I₆₀ were 28.0%, 23.0% and 19.1% higher than those in CK, respectively. Except for D₆₀, the FNC/POC and MNC/POC ratios were significantly lower than CK under other treatments. In terms of POC fractions, the MNC/POC ratios of D₄₀, D₂₀, I₂₀, I₄₀ and I₆₀ were 28.4%, 23.3%, 28.8%, 23.3% and 32.2% lower than that of CK, respectively. The FNC/POC ratio of D₄₀, D₂₀, I₂₀, I₄₀ and I₆₀ was found to be lower by 23.3%, 16.1%, 21.0%, 27.0% and 31.0% compared to that of CK, respectively. 4) NH₄⁺-N and pH were the primary factors influencing the content of MNC in different carbon fractions under varying precipitation conditions. In summary, alterations in precipitation (either increase or decrease) enhanced the contribution of BNC-dominated MNC to MAOC, but reduced the contribution of FNC-dominated MNC to POC. This study was of significance for understanding the distribution of microbial necromass across different organic carbon fractions under precipitation alterations.