Response of soil phosphorus fractions to land use change in the subalpine ecosystems of Southeast margin of Qinghai-Tibet Plateau, Southwest China

Land use change has a significant effect on soil nutrient cycling. However, data on soil phosphorus (P) fractions following land use change are scarce, especially in the subalpine ecosystem of China. Here, we investigated the influence of land use change on soil P fractions in the subalpine ecosystem of the Southeast margin of Qinghai-Tibet Plateau, Southwest China. In total, we took 168 soil samples from four land use types, including natural forest (NF), artificial forest (AF), farmland (FL), and shrubland (SL), at a depth of 0–20 and 20–40 cm. Phosphorus fractions were determined using Hedley fractionation procedures. The results showed that land use change and soil depths had a significant impact on the status of P fractions (p≤0.05). Organic P (Po) fraction significantly decreased following land use change but varied among land use types. Farmland soils showed the highest losses of Po fractions ranging from 23 to 54%, due to organic material reduction. Total P also decreased by 9.63% after the conversion of NF to AF (p≤0.05). Conversely, total P and inorganic P (Pi) fractions were significantly higher in FL soil than in other land uses (p≤0.05), likely due to the application of chemical fertilizers. Organic P extracted by NaOH was the dominant total Po fraction in all land uses, accounting for 63.3 to 73.14% of total Po, while Pi extracted by HCl was the dominant Pi fraction in FL soil, accounting for 42.74% of total P. Soil moisture content, pH, soil organic carbon (SOC), total nitrogen (total N), and calcium were key soil properties governing soil P fractions. Organic P fractions were strongly positively correlated with SOC and total N, implying that soil organic matter (SOM) plays a vital role in maintaining the soil P reserves. Generally, our findings shed light on how land use changes significantly affected P fractions. This result may have substantial implications for soil P management to maintain soil P reserves by reducing SOM losses.