Soil C and N cycling in three semiarid vegetation types: Response to an in situ pulse of plant detritus

Plant detritus is an important source of labile C that drives soil microbial growth and regulates the balance of N mineralization and immobilization. In semiarid ecosystems, timing of plant detrital inputs may be especially important in regulating microbial C and N cycling because of the relatively short window of time when moisture is available. Low soil moisture in early-summer may inhibit microbial colonization of recently released detritus, resulting in C-limitations to microbial growth, and this may explain the NO3− accumulation commonly observed in semiarid, arid, and Mediterranean ecosystems. We examined linkages between soil C availability and gross N cycling rates during summer in three common semiarid vegetation types: sagebrush, crested wheatgrass, and cheatgrass. To determine whether dry soils inhibit microbial colonization of plant detrital inputs, we stimulated soil C availability in situ by killing plant biomass shortly before the summer dry-season with herbicide (detrital-pulse treatment). Soil C and gross N cycling rates were determined during field incubations of intact soil cores from untreated soils on three occasions from late-spring to late summer, and from detrital-pulse treated soils on two occasions in summer. We hypothesized that greater C availability, resulting in increased microbial biomass and C mineralization rates, would translate to greater N immobilization rates, and this would inhibit the accumulation of inorganic N during summer months.There were few differences in soil C and N cycling among vegetation types. In all vegetation types, the in situ detrital-pulse stimulated soil C mineralization and gross N cycling rates compared to untreated soils; however, this treatment did not inhibit the summertime accumulation of NO3−. Instead, elevated N cycling rates and large labile N pools in detrital-pulse soils persisted throughout the summer. Our results combined with a model of microbial C–N dynamics indicate that microbes in detrital-pulse soils were utilizing substrates with C:N ratios 27% lower than in untreated soils (p < 0.04), and much lower than expected based on the C:N of plant detritus. This suggests that substrates released by senescing plants had much lower C:N than would be predicted based on the overall C:N of plant tissue. In addition, appearance of 15N in different soil density fractions showed that the detrital-pulse treatment stimulated microbial N immobilization in both C-rich and N-rich soil microsites. Greater N immobilization associated with light fraction organic matter is consistent with greater microbial growth due to earlier input of plant detritus. Interestingly, heavy fraction organic matter was also an important sink for immobilized N and was strongly stimulated by the detrital-pulse treatment, indicating that this fraction is not as recalcitrant as formerly thought.