Conversion of alpine pastureland to artificial grassland altered CO2 and N2O emissions by decreasing C and N in different soil aggregates

Background The impacts of land use on greenhouse gases (GHGs) emissions have been extensively studied. However, the underlying mechanisms on how soil aggregate structure, soil organic carbon (SOC) and total N (TN) distributions in different soil aggregate sizes influencing carbon dioxide (CO2), and nitrous oxide (N2O) emissions from alpine grassland ecosystems remain largely unexplored. Methods A microcosm experiment was conducted to investigate the effect of land use change on CO2and N2O emissions from different soil aggregate fractions. Soil samples were collected from three land use types, i.e., non-grazing natural grassland (CK), grazing grassland (GG), and artificial grassland (GC) in the Bayinbuluk alpine pastureland. Soil aggregate fractionation was performed using a wet-sieving method. The variations of soil aggregate structure, SOC, and TN in different soil aggregates were measured. The fluxes of CO2 and N2O were measured by a gas chromatograph. Results Compared to CK and GG, GC treatment significantly decreased SOC (by 24.9–45.2%) and TN (by 20.6–41.6%) across all soil aggregate sizes, and altered their distributions among soil aggregate fractions. The cumulative emissions of CO2 and N2O in soil aggregate fractions in the treatments of CK and GG were 39.5–76.1% and 92.7–96.7% higher than in the GC treatment, respectively. Moreover, cumulative CO2emissions from different soil aggregate sizes in the treatments of CK and GG followed the order of small macroaggregates (2–0.25 mm) > large macroaggregates (> 2 mm) > micro aggregates (0.25–0.053 mm) > clay +silt (< 0.053 mm), whereas it decreased with aggregate sizes decreasing in the GC treatment. Additionally, soil CO2 emissions were positively correlated with SOC and TN contents. The highest cumulative N2O emission occurred in micro aggregates under the treatments of CK and GG, and N2O emissions among different aggregate sizes almost no significant difference under the GC treatment. Conclusions Conversion of natural grassland to artificial grassland changed the pattern of CO2 emissions from different soil aggregate fractions by deteriorating soil aggregate structure and altering soil SOC and TN distributions. Our findings will be helpful to develop a pragmatic management strategy for mitigating GHGs emissions from alpine grassland.

Diversity-dependent stability under sowing and nutrient addition: evidence from non-weeded artificial grassland

A growing body of evidence from diversity-manipulation and natural studies suggests that the stability of community productivity increases with biodiversity; however, few studies have researched this relationship in a non-weeded grassland. To clarify this issue, we established an artificial grassland in 2003 using three common species, Elymus nutans, Festuca sinensis and Festuca ovina, which included seven different community structures (three monocultures, three two-species mixtures and one three-species mixture based on sown species) and two nutrient addition treatments (non-nutrient addition and nutrient addition). Data was collected over a three-year period (2011–2013). Our results showed that the sowing species modified realized species richness (i.e. the number of total species we observed in a community) and species evenness, but had negligible influences on community- and population-level stability. Furthermore, all of these variables were reduced by nutrient addition. These dynamics did not alter the positive influence of realized species richness on community stability, but restricted the stable effect of evenness because this effect was only significant under nutrient addition condition. The potential mechanisms underlying these processes were statistical averaging and species asynchrony, rather than overyielding effect. Conversely, population stability decreased with realized species richness in non-nutrient addition treatments. We conclude that biodiversity contributed to community- and population-level stability even in non-weeded experiment. This process resulted from different mechanisms that observed in weeded experiments. Further studies in other ecosystems (e.g. aquatic ecosystem) are needed to find a more general conclusion.