Hua Fu

Grazing exclusion alters soil microbial respiration, root respiration and the soil carbon balance in grasslands of the Loess Plateau, northern China

Abstract Grassland ecosystems are a significant component of the global carbon cycle. To better understand how grazing affects the carbon cycle of grasslands, soil microbial respiration (Rm) and root respiration (Rr), which are the main soil respiration components, we investigated with a trenching method in grazed grasslands (GG) and fenced (FG) grasslands on the Loess Plateau, northern China in 2008. The annual carbon balance in the two grasslands were also assessed and compared. After exclusion of grazing for about 3 years, soil organic carbon (SOC) and microbial biomass carbon (MBC) in the surface soil increased significantly (P < 0.05), resulting in the increase of Rm in most seasons. Exclusion of grazing did not change the diurnal variations of Rm, Rr and total soil respiration (Rt). Grazing decreased the temperature dependence of Rm. The annual accumulations of Rm were 165.9 g C m−2 in FG and 116.1 g C m−2 in GG. On most dates, Rr in FG was higher than in GG, but significant differences were only found in some seasons. The seasonal average value of Rr was 0.374 µmol carbon dioxide (CO2) m−2 s−1 in FG, 21.0% higher than that in GG (0.309 µmol CO2 m−2 s−1). Net primary production (NPP) in FG and GG were 243.6 and 205.8 g C m−2, respectively. The annual C balance resulted in net C sequestrations of 77.7 and 89.7 g C m−2 in FG and GG, respectively, suggesting that the grassland in this region may act as a C sink both under grazing and fencing.

Effects of land‐use regimes on carbon sequestration in the Loess Plateau, northern China

Abstract The effects of different land‐use regimes on C sequestration in the terrestrial ecosystems were investigated in the Loess Plateau. We found that the surface soil (0–10 cm) was more active at sequestering carbon after land had been used as grassland. When cropland was converted to grassland, C storage in surface soil (0–10 cm) increased. Grazing exclusion led to vegetation and soil restoration, thus increasing the sequestration of atmospheric C. Intensive cultivation led to a decline of soil organic carbon content in the surface soil of cropland. Long‐term application of organic manure accompanied with moderate chemical fertiliser can greatly enhance the C storage in the soil layer below the tilled layer in cropland. In this region, both grassland and cropland can contribute to C sequestration when proper management practice is implemented.

Grazing exclusion alters ecosystem carbon pools in Alxa desert steppe

The Alxa desert steppe has been strongly degraded by overgrazing, contributing c. 22% of the total springtime dust originating from Asia. Previous work in this region has focused on the impacts of grazer exclusion on restoration of vegetation and soil fertility, yet carbon dynamics are not well known. The effects of 7 years of grazer exclusion on carbon dynamics were studied and related to changes in vegetation and soil properties. Removal of grazing resulted in a significantly greater plant cover and aboveground plant biomass compared with areas that had been subject to grazing, but this had no effects on belowground plant biomass. Removal of grazing resulted in significantly decreased soil bulk density in the 0–10 cm layer, increased soil water content (7% cf. 40%) and greater soil microbial biomass C (6% cf. 73%) compared with soils in the grazed area. Soil organic carbon (SOC) pools were lower and soil inorganic carbon (SIC) pools were higher in areas that were excluded from grazing. After 7 years of grazer exclusion, the total C pool in the plant–soil system was 10% greater (primarily due to 21% greater in SIC) than that in the area that had been grazed over that time period.

Conventional tillage increases soil microbial biomass and activity in the Loess Plateau, China

Land-use change can affect the quantity and quality of soil organic matter (SOM) as well as soil microbial biomass and activity. Land use in the Loess Plateau has undergone great changes in the past five decades. To understand the effects of these changes on soil chemical and biological properties in the Loess Plateau, we determined soil organic carbon (SOC), total nitrogen (TN), soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN), and basal respiration in millet cropland (MC), grazed grassland (GG) and enclosed grassland (EG). Grazing exclusion led to an increase of SOC and TN. Compared with the MC, the two grasslands had higher SOC and TN values in the 0–10 cm layer, but lower values in the 20–30 cm layer. SMBC and SMBN values in the EG were intermediate between the MC and GG. Conversion of cropland to grassland increased soil basal respiration (SR) and metabolic quotient (qCO2), while grazing exclusion resulted in higher SR but lower qCO2. The study suggests that grassland enclosure could increase SOM as well as soil microbial biomass and activity, and conventional cropland might be the optimal land-use type to maintain soil chemical and biological properties in the Loess Plateau, China.

The impact of nitrogen enrichment on grassland ecosystem stability depends on nitrogen addition level

Increasing atmospheric nitrogen (N) deposition may affect plant biodiversity, subsequently altering ecosystem stability. While a few studies have explored how simulated N deposition affects community stability and its underlying mechanisms, the experimental levels of N addition used are usually higher than current and future N deposition rates. Thus, their results could produce highly uncertain predictions of ecosystem function, especially if the responses to N deposition are nonlinear. We conducted a manipulative experiment that simulated elevated atmospheric N deposition with several N addition levels to evaluate the effect of N deposition on ecosystem stability and its underlying mechanisms in a semiarid grassland in northern China. Here we show that N addition altered community diversity, reducing species richness, evenness, diversity and dominance. In addition, we found that N addition at current N deposition levels had no significant impact on community stability. In contrast, N addition at levels from 4.6 to 13.8gNm-2yr-1 significantly decreased community stability, although community stability for the 13.8gNm-2yr-1 treatment was higher than that for the 4.6gNm-2yr-1 treatment. These results indicate that the response of community stability to N enrichment is nonlinear. This nonlinear change in community stability was positively correlated with species asynchrony, species richness, and species diversity as well as the stability of dominant species and the stability of the grass functional group. Our data suggest a need to re-evaluate the mechanisms responsible for the effects of N deposition on natural ecosystem stability across multiple levels of N enrichment and that additional experimentation with gradients of N loads more similar to future atmospheric N deposition rates is needed.

Variation of Q10 values in a fenced and a grazed grassland on the loess plateau, northwestern China

Abstract Soil respiration (SR) and microbial respiration (MR), which were primarily regulated by soil temperature, can act as a feedback to climate change. Although many studies suggest that global warming will accelerate carbon dioxide (CO2) emissions from soil, the magnitude of this feedback is unknown, mostly due to uncertainty in the temperature sensitivity (Q10, increaing ratio of SR and MR after a 10°C increase of temperature) of SR and MR. To investigate the seasonal variation of short-term Q10 and estimate how grazing impacts temperature sensitivity of SR and MR, we measured SR and MR in a fenced and a grazed grassland on the loess plateau, northwestern China, during 2008–2010. In this semiarid grassland ecosystem, soil temperature was the dominant factor controlling SR and MR during the experimental period. Short-term apparent Q10 of SR and MR had a clearly seasonal variation, and was significantly and negatively related to soil temperature at 2-cm depth. However, no relationship was found between soil moisture (0–10 cm soil layer) and apparent Q10. By decreasing soil organic carbon and root biomass, grazing reduced the long-term Q10 of SR and MR. In both the short term and the long term, Q10 of MR was lower than that of SR, suggesting that autotrophic respiration is more sensitive than heterotrophic respiration to temperature. We emphasize the importance of using Q10 when modeling trajectories of soil carbon stocks under climate change scenarios, and the seasonal variation of Q10 should also be regarded as a parameter in the global carbon models.

Contribution of root respiration to total soil respiration in a semi-arid grassland on the Loess Plateau, China

Using the trenching method, a study was conducted in a grassland on the Loess Plateau of northern China in 2008 and 2009 to partition total soil respiration (Rt) into microbial respiration (Rm) and root respiration (Rr). Using the measurements of soil CO2 diffusivity and soil CO2 production, an analytical model was applied to correct the data, aiming to quantify the method-induced error. The results showed that Rm and Rr responded differently to biotic and abiotic factors and exhibited different diurnal and seasonal variations. The diurnal variation of Rm was strongly controlled by soil temperature, while Rr might be mainly controlled by photosynthesis. The combination of soil temperature and moisture could better explain the seasonal variation in Rm (r2=0.76, P<0.001). The seasonal variation of Rr was influenced mainly by the plant activity. The contribution of root respiration to total soil respiration (Rr/Rt ratio) also exhibited substantial diurnal and seasonal variations, being higher at nighttime and lower at daytime. In the different growing stages, the Rr/Rt ratios ranged from 15.0% to 62.0% in 2008 and 14.5% to 63.6% in 2009. The mean values of the Rr/Rt ratio in the growing season and the annual mean Rr/Rt ratio were 41.7% and 41.9%, respectively, during the experiment period. Different precipitation distributions in the two years did not change the yearly Rr/Rt ratio. Corrected with the analytical model, the trenching method in small root-free plots led to an underestimation of Rr and Rr/Rt ratio by 4.2% and 1.8%.

Conventional tillage improves the storage of soil organic carbon in heavy fractions in the Loess Plateau, China

Soil labile organic carbon (C) plays an important role in improving soil quality. The relatively stable fractions of soil organic C (SOC) represent the bulk of SOC, and are also the primary determinant of the long-term C balance of terrestrial ecosystems. Different land use types can influence the distribution patterns of different SOC fractions. However, the underlying mechanisms are not well understood. In the present study, different fractions of SOC were determined in two land use types: a grazed grassland (established on previously cultivated cropland 25 years ago, GG) and a long-term cultivated millet cropland (MC). The results showed that C concentration and C storage of light fractions (LF) and heavy fractions (HF) presented different patterns along the soil profiles in the two sites. More plant residues in GG resulted in 91.9% higher LF storage at the 0–10 cm soil depth, further contributed to 21.9% higher SOC storage at this soil depth; SOC storage at 20–60 cm soil depth in MC was 98.8% higher than that in GG, which could be mainly attributed to the HF storage 104.5% higher than in GG. This might be caused by the long-term application of organic manure, as well as the protection from plough pan and silt- and clay-sized particles. The study indicated that different soil management practices in this region can greatly influence the variations of different SOC fractions, while the conventional tillage can greatly improve the storage of SOC by increasing heavy fractions.