Contrasting effects of maize litter and litter-derived biochar on the temperature sensitivity of paddy soil organic matter decomposition

Organic matter input regulates the rate and temperature sensitivity (expressed as
Q
10
) of soil organic matter (SOM) decomposition by changing microbial composition and activities. It remains unclear how the incorporation of litter-made biochar instead of litter affects the
Q
10
of SOM decomposition. Using a unique combination of two-and three-source partitioning methods (isotopic discrimination between C3/C4 pathways and
14
C labeling), we investigated: (1) how maize litter versus litter-made biochar (of C4 origin) addition influenced the
Q
10
of SOM (C3 origin) under 10°C warming, and (2) how the litter or biochar amendments affected the
Q
10
of
14
C-labeled fresh organic matter (FOM) after long-term incubation. Compared with biochar addition, litter increased the rates and
Q
10
of mass-specific respiration, SOM and FOM decomposition, as well as the contents of SOM-derived dissolved organic C (DOC) and total phospholipid fatty acids (PLFA). Litter-amended soils have much higher activities (
V
max
) of β-glucosidase, N-acetyl-β-glucosaminidase, and leucine aminopeptidase, suggesting larger enzyme pools than in soils with biochar. The
Q
10
of enzyme
V
max
(1.6–2.0) and
K
m
(1.2–1.4) were similar between litter-and biochar-amended soils, and remained stable with warming. However, warming reduced microbial biomass (PLFA) and enzyme activity (
V
max
), suggesting decreased enzyme production associated with smaller microbial biomass or faster enzyme turnover at higher temperatures. Reductions in PLFA content and enzyme
V
max
due to warming were larger in litter-amended soils (by 31%) than in the control and biochar-amended soils (by 4–11%), implying the active litter-feeding microorganisms have a smaller degree of heat tolerance than the inactive microorganisms under biochar amendments. The reduction in enzyme activity (
V
max
) by warming was lower in soils with biochar than in the control soil. Our modeling suggested that the higher
Q
10
in litter-amended soils was mainly caused by faster C loss under warming, linked to reductions in microbial biomass and growth efficiency, rather than the slightly increased SOM-originated substrate availability (DOC). Overall, using straw-made biochar instead of straw
per se
as a soil amendment lowers the
Q
10
of SOM and FOM by making microbial communities and enzyme pools more temperature-tolerant, and consequently reduces SOM losses under warming.