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.