3.4 Effects of N and P addition on GRSP and SOC concentrations
T-GRSP and EE-GRSP contents were determined after N, P, NP, and control treatments (Fig. 5). Similarly, the mean concentrations of T-GRSP and EE-GRSP were significantly different (P < 0.05) between the surface and subsurface soils. However, T-GRSP and EE-GRSP contents were not significantly different between the surface and subsurface soils after N, P, NP, and the control treatments for five years. Short-term nutrient addition did not affect the soil GRSP. Meanwhile, the SOC content was also not significantly affected. Furthermore, SOC was positively correlated with both T-GRSP and EE-GRSP (P<0.001) (Fig. 6). These results show that short-term N and P addition cannot improve soil organic carbon pool.
4. Discussion
4.1 Grazing and GRSP
Livestock grazing is the major human activity in grassland ecosystems that alters grassland production, biodiversity, soil C storage and ultimately causes grassland degradation (Wang et al., 2020; Liu et al., 2021). This study used Kobresia meadow, the main type of alpine meadow widely distributed across the middle and eastern part of the Qinghai-Tibetan Plateau (He et al., 2016), and causes various degenerate successions. Gramineae-Kobresia humiliscommunity is a typical alpine meadow that is made up of a double layer structure (upper layer, gramineous forage grass, and lower layer,Kobresia humilis community). However, long-term overgrazing affects gramineous forage growth and reproduction due to livestock feeding and trampling. The sedge plants of the root system rapidly grow, increasing the root-soil ratio and root cap ratio. Meanwhile, the mattic epipedon cracks due to the dry root environment, corrosion caused by harsh weather, and disturbance by the small mammalian herbivores, thus gradually degrading the alpine meadow (Cao et al., 2018). Therefore, the alpine meadow successively undergoes the degradation and succession process (GK, K, KT, KC, and FB). Herein, the aboveground plant biomass and diversity were decreased with the intensification of grazing. These results show that soil properties change with the grazing gradients. For instance, the soil water and nutrient content decreased from the GK community to the FB community and thus affecting the C and GRSP contents. GRSP contents were gradually decreased with increased grazing intensities. Chen et al. (2018) also indicated that GRSP in soil (2–15 mg/g) contains 5–10% of total SOC. Herein, GRSP concentrations were also within the above range with 3.7-14.18% SOC. GRSP is a recalcitrant compound and a stable form of SOC (Chen et al., 2018; Wang et al., 2018). However, disturbance from outside environments affects GRSP (Zhang et al., 2015; Lozano et al., 2016). Grazing affects GRSP stability and the fixation of soil organic matter in alpine grassland. Moreover, the succession process of alpine grassland causes various soil organic carbon succession.