The effect of land use on soil fertility and phosphorus dynamics in sub-alpine grassland soils of Gansu, China
The objectives of this study were (1) to determine if there is any soil degradation as indicated by soil fertility declines and soil erosion; (2) to understand soil P dynamics under different land uses; and (3) to determine if pasture degradation can be evaluated by plant cover, plant species changes, and soil erosion. This study, conducted on sub-alpine grasslands in Gansu, China, deals with the impacts of land use on soil fertility, P dynamics, soil erosion, and above ground vegetation. Land uses include three magnitudes of pasture degradation, lightly (LDGP), moderately (MDGP), and heavily degraded pasture (HDGP), and cultivated fields varying 1 to 50 years of cultivation. Soil samples were collected from 18 sites at seven locations, from either Chemozemic or Castanozemic (Chestnut) soils lying between 2,600 to 3,000 in above sea level (ASL). To determine if there were any influences of pasture degradation and cultivation on soil fertility and soil erosion, nutrients-associated with macro-organic matter,total N, P, K, CEC, pH and EC were analyzed. The activity of 137Cs was determined to estimate soil erosion as well. In HDGP, soil CEC, total C and total N dropped by 18%, 33%, and 28%, respectively, on a regional scale compared to LDGP. Furthermore, cultivating grasslands significantly decreased soil CEC, organic C and total N by 21%, 59% and 52%, respectively, after 30 to 50 years' cultivation. Soil pH also significantly increased with longer cultivation. With soil degradation, soil EC increased in previously saline soils, but decreased in non-saline soil. Cultivation also increased the proportion of water-soluble Na from 7% in LDGP to 22% of total soluble cations after 41 years' cultivation at Tianzhu-A/B. Soil erosion and mineralization of organic matter were responsible for lower soil CEC, organic C, total N, and soil macro-organic matter. The concentration of 137CS was significantly reduced when pasture was heavily degraded, or was put into crop production. More than half of 137Cs activity in soil was lost with 30 to 50 years' cultivation compared to LDGP. Topography and climatic differences had a great influence on soil erosion. Pasture degradation, cultivation and erosion also caused changes in soil P dynamics. Phosphorus fractionation showed that more labile P was found in LDGP compared to HDGP. Cultivation and fertilization significantly raised labile P and Ca-Pi levels. Mineralization of organic P, incorporation of sub-soil by tillage following erosion, and fertilization were major sources of Ca-Pi in cultivated fields. In general, Fe- and Al-associated Pi was higher after pasture was cultivated. Soil organic P, especially the moderately labile fraction extracted by NaOH, declined with pasture degradation and cultivation. Once pasture became heavily degraded, changes in plant species composition and plant cover were observed. A 99% plant cover was found in LDGP, while the lowest plant cover of 62% was observed in HDGP. The plant palatability index (PI) was developed in this study based on numbers of individual plants inside of the quadrat and their assigned numerical values. Results showed that the PI decreased from 205 in LDGP to 173 in MDGP, and then to 151 in HDGP, implying that the abundance of more palatable plants may decrease with pasture degradation, while plants with lower forage value tended to increase. Research results were extrapolated to the whole area between 2,600 to 3,000 m ASL in Gansu based on area estimation and land use patterns. Grassland was the major land use, accounting for 85% of the total. Cultivated land occupied only 3.5% of the total, but the latter was vital to local farmers for food and feed production. Lands between 2,600 to 3,000 m ASL had a great potential for soil degradation. About half of the 1.9 million ha grassland had been either moderately or heavily degraded, resulting in 22.7 and 1.8 million tons of C and N losses, respectively. Potentially available P loss reached 25.8 thousand tons on grassland. Cultivating 80,000 ha of grasslands resulted in 3.8 million tons of C, 0.31 million tons of N and 11.6 thousand tons of P losses in the region.