Legacy effects of grazing and nitrogen fertilization on soil carbon, nitrogen and phosphorus in an alpine meadow on the Qinghai-Tibetan Plateau

Increasing soil carbon (C) and nitrogen (N) storage can help mitigate climate change and sustain soil fertility. Changes in herbivore and anthropogenic nutrient enrichment intensities can lead to dramatic shifts in the plant and microbial communities, soil organic carbon (SOC) and nutrient dynamics. However, the legacy effects of grazing and N enrichment on the biogeochemical processes remain unclear. Here, we conducted a 6-year rotational grazing (Stocking rates: 0, 8 and 16 sheep ha⁻¹) and 4-year N-addition (N addition levels: 0, 50, 100 and 200 kg N ha⁻¹ yr⁻¹) experiment to investigate how soil C, N and phosphorus (P) components respond to the legacy effects of grazing and N fertilization after a 3-year cessation of grazing and N addition treatments in an alpine meadow on the Qinghai-Tibetan Plateau (QTP). We show that previous grazing significantly increased soil total nitrogen (STN), slightly increased SOC and decreased soil total phosphorus (STP); while previous N fertilization significantly decreased SOC, but it did not significantly alter STN and STP. Previous grazing at low stocking rates (≤ 8 sheep ha⁻¹) might amplify the negative legacy effects of N fertilization on SOC, while a higher stocking rate would weaken the negative impacts of previous N fertilization on SOC. The interactive and synergistic impacts of historical grazing and N fertilization induced a significantly negative effect on STP. Previous N fertilization decreased soil microbial carbon (MBC) and increased soil available N:P, resulting in the reduction of SOC. The increase in plant diversity caused by previous grazing increased SOC, which counteracted the negative effects of increasing bacterial diversity. Previous grazing-induced decreasing bacterial community heterogeneity may lead to increased STN. Although previous grazing-induced increases in soil moisture and soil nutrient availability may have positive effects on STP, previous grazing-induced negative effects on STP may exceed those positive effects. Therefore, the legacy effects of grazing could be beneficial for improving soil C and N, but may increase the risk of soil P loss in the short term, while residual exogenous N could pose a detrimental effect on C storage over time. Reintroducing grazing and/or P addition may be an appropriate choice to offset the adverse consequence of N deposition in the context of global change. Our findings suggest that the stocking rate at about 8 sheep ha⁻¹ could be a suitable grassland management technique for soil fertility sequestration and mitigating the negative influences of residual exogenous N in the QTP.