Fencing enclosure alters nitrogen distribution patterns and tradeoff strategies in an alpine meadow on the Qinghai-Tibetan Plateau

Abstract

The grasslands of Qinghai-Tibetan Plateau have become extremely degraded, resulting in widespread deficiency of soil N. In efforts to restore degraded lands, fencing enclosure has been used extensively. However, the effect of fencing on N allocation patterns and nutritional strategy of alpine plants are equivocal. In this study, we used 15N tracer (CO (15NH2)2, 10 g N m−2) to examine the allocation and distribution of N in plants and soil in grasslands either grazed heavily by livestock or fenced for three years in an alpine meadow of Northern Tibet. The 15N recovery (15Nrec) in shoots of the fenced enclosure increased by 207% in grasses, decreased by 103% in forbs, and did not change in sedges when compared to the grazed meadow. The 15Nrec in shoots accounted for only 1.97% and 4.65% of the total N in the grazed and fenced meadows, respectively. Fencing increased soil 15N content at 0–5 cm depth by 6.9%, but decreased the content at 5–10 cm depth by 11.7%. The results demonstrated that fencing altered the soil N distribution by increasing 15Nrec in top soil and by decreasing 15Nrec in subsurface soil. In addition, fencing had no impact on root 15N storage (33%–39%), 15N losses (9.6%–12.5%) and soil available 15Nrec (NH4+-N, NO3–-N and light fraction organic N), but decreased root:shoot 15Nrec ratio by 49.8% (16.9:1 in grazed and 8.4:1 in fenced grassland). Fencing increased soil organic carbon, total N and NO3–-N concentrations, which indicated that the strategy of the plants was to allocate relatively more N to roots in nutrient-poor soil (grazed) but relatively more N to shoots in nutrient-rich soil (fenced).