Changes in soil organic carbon, total nitrogen and total phosphorus in 2000–2020 and their driving mechanisms in Tibetan alpine grasslands

Soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) and their ratios are important parts of global carbon, nitrogen and phosphorus cycle, but their responses to climate change and human activities remain controversial in alpine grasslands of the Tibetan Plateau. In this study, we quantified the influences of climate change and human activities on SOC, TN, TP and their ratios at three depths (0–10, 10–20 and 20–30 cm) during 2000–2020 across the Tibetan alpine grasslands. Under the combined influences of climate change and human activities, spatially average SOC, TN, TP, C:N, C:P and N:P at 0–10 cm, and C:N and C:P at 10–20 cm decreased by 3.42%, 1.35%, 0.99%, 0.39%, 1.75%, 0.51%, 0.03% and 0.88%, respectively, but the others increased by 0.01–3.26%. Under the single influence of climate change, spatially TP at 0–10 and 20–30 cm decreased by 0.30% and 1.05%, respectively, but the others increased by 0.14–4.35%. Under the single influence of human activities, spatially TN at 0–10 and 10–20 cm, C:P at 0–10 cm and N:P at 10–20 cm decreased by 15%, 1%, 7% and 4%, respectively, but the others increased by 1–157%. No matter under the double pressure of climate change and human activities or their individual pressure, the changes of SOC, TN, TP, C:N, C:P and N:P not always increased or decreased, but vary with longitude, latitude and elevation. The changes of SOC, TN, TP, C:N, C:P and N:P did not necessarily and linearly increase or decrease with their background values. Compared to precipitation change and warming, radiation change had a stronger influence on the change of SOC at 10–20 cm and TP at 20–30 cm under the double pressure of climate change and human activities, and the single pressure of human activities, respectively. Radiation change also had stronger influences on the change of SOC at 10–20 cm, TN at 0–10 cm, C:N at 20–30 cm, C:P at 0–10 and 10–20 cm, N:P at 0–10 and 10–20 cm under the single pressure of climate change. Therefore, in the past 20 years, the single effect of climate change or human activities on soil organic carbon, total nitrogen, total phosphorus and their ratios underestimated or overestimated their interactive effects in the Tibetan grasslands. Climate change and human activities reconstructed the spatial distribution patterns of soil organic carbon, total nitrogen, total phosphorus and their ratios. Radiation change should be given the same importance as warming and precipitation change, at least for the change of soil organic carbon, total nitrogen and total phosphorus of the Tibetan alpine grasslands.