Litter removal effectively alleviates the nitrogen limitation in grazing exclusion alpine meadows

Abstract

Litter accumulation in the alpine meadows after long-term grazing exclusion hinders the restoration efforts of degraded or mismanaged grassland ecosystems. Little is still known about how removing the litter affects the soil’s nutrient cycling and microbial metabolism. Here, we sampled soils in alpine meadows after four years grazing exclusion along a mass gradient of litter removal (MGLR) and a time gradient of litter removal (TGLR) in the northeastern region of the Qinghai-Tibet Plateau, China. Five soil extracellular enzyme activity (EEA, including carbon-, nitrogen-, and phosphorus- acquiring enzymes), and soil environmental variables were measured, and microbial metabolic limitation was calculated using vector analysis. The results indicated that short interval TGLR had no significant effect on soil enzyme activity. Meanwhile, the MGLR increase eventually promoted the activity of the soil C-, N-, and P-acquiring enzymes, which was beneficial for nutrient cycling. Microbial C limitation displayed the maximum value (1.47) after 50% removal of the litter. Litter removal effectively relieved the pressure on N limitation. The study further provided evidence that the dissolved organic carbon-to-soil available nitrogen ratio (DOC:SAN) and the soil available nitrogen-to-soil available phosphorus ratio (SAN:SAP) were the main environmental factors influencing microbial C limitation and nutrient limitation, respectively. The microbial C (1.33–1.35) and N (34.00°­34.09°) limitation were relatively low when the MGLR was 70%-90%, which was conducive to the element balance and efficient utilization of soil nutrient resources. Altogether, these findings indicated that litter removal had a significant impact on enhancing the activity of EEA and alleviating nutrient limitation despite the other unknown drawbacks. Litter removal and increasing the number of nitrogen-fixing plants may be the effective methods at present to alleviate nitrogen limitation in alpine meadows.