Nitrogen deposition alters nitrogen cycling and exotic annual grass litter decomposition in southern California.

摘要

This study demonstrated that nitrogen (N) deposition alters litter decomposition and N cycling in exotic annual grasslands that exist in former coastal sage scrub ecosystems of southern California. N deposition resulted in altered tissue chemistry of the exotic annual grass Bromus diandrus by increasing tissue N and decreasing lignin content. Altered B. diandrus tissue chemistry and elevated soil N associated with N deposition resulted in accelerated rates of N movement from litter to soil and litter decomposition during the early stages of decomposition. Microbial enzyme activities were used as indicators of microbial response to litter quality and soil N. Faster mass loss rates of high N/low lignin litter compared to low N/high lignin litter corresponded to higher activity levels of the C acquiring enzyme, beta-1,4 glucosidase. Faster N mineralization from high N/low lignin litter is probably due to greater activity of the proteolytic enzyme leucine aminopeptidase. As litter decomposition progressed, changes in microbial enzyme activity from decomposing B. diandrus litter reflected changes in tissue carbon (C) and N. By the later stages of decomposition, there was no longer a difference in mass loss between litter types. N deposition resulted in faster rates of gross and net soil N mineralization during the beginning of the rainy season, which could make N available for plant uptake earlier than without N deposition. Long-term N deposition resulted in N accumulation in the soil profile to at least 1 m, which could allow N to persist in the ecosystem. By altering ecosystem processes in exotic annual grasslands of southern California, N deposition may facilitate the invasion of nitrophilous exotic annual grasses into native coastal sage scrub.