Decrease in soil pH has greater effects than increase in above‐ground carbon inputs on soil organic carbon in terrestrial ecosystems of China under nitrogen enrichment

Mowing aggravates the adverse effects of nitrogen addition on soil acid neutralizing capacity in a meadow steppe

Soil acidification induced by reactive nitrogen (N) inputs is a major environmental issue in grasslands, as it lowers the acid neutralizing capacity (ANC). The specific impacts of different N compound forms on ANC remain unclear. Grassland management practices like mowing and grazing can remove a considerable amount of soil N and other nutrients, potentially mitigating soil acidification by removing N from the ecosystem or aggravating it by removing base cations. However, empirical evidence regarding the joint effects of adding different forms of N compounds and mowing on ANC changes in different-sized soil aggregates is still lacking. This study aimed to address this knowledge gap by examining the effects of three N compounds (urea, ammonium nitrate, and ammonium sulfate) combined with mowing (mown vs. unmown) on soil ANC in different soil aggregate sizes (>2000 μm, 250-2000 μm, and <250 μm) through a 6-year field experiment in Inner Mongolia grasslands. We found that the average decline in soil ANC caused by ammonium sulfate (AS) addition (-78.9%) was much greater than that by urea (-25.0%) and ammonium nitrate (AN) (-52.1%) as compared to control. This decline was attributed to increased proton (H+) release from nitrification and the leaching of exchangeable Ca2+ and Mg2+. Mowing aggravated the adverse effects of urea and AN on ANC, primarily due to the reduction in soil organic matter (SOM) contents and the removal of exchangeable Ca2+, K+, and Na + via plant biomass harvest. This pattern was consistent across all aggregate fractions. The lack of variation in soil ANC among different soil aggregate fractions is likely due to the contrasting trend in the distribution of exchangeable Ca2+ and Mg2+. Specifically, the concentration of exchangeable Ca2+ increased with increasing aggregate size, while the opposite was true for that of exchangeable Mg2+. These findings underscore the importance of considering the forms of N compounds when assessing the declines of ANC induced by N inputs, which also calls for an urgent need to reduce N emissions to ensure the sustainable development of the meadow ecosystems.

Negative correlation between soil salinity and soil organic carbon variability

Soil organic carbon (SOC) is vital for terrestrial ecosystems, affecting biogeochemical processes, and soil health. It is known that soil salinity impacts SOC content, yet the specific direction and magnitude of SOC variability in relation to soil salinity remain poorly understood. Analyzing 43,459 mineral soil samples (SOC < 150 g kg-1) collected across different land covers since 1992, we approximate a soil salinity increase from 1 to 5 dS m-1 in croplands would be associated with a decline in mineral soils SOC from 0.14 g kg-1 above the mean predicted SOC ([Formula: see text] = 18.47 g kg-1) to 0.46 g kg-1 below [Formula: see text] (~-430%), while for noncroplands, such decline is sharper, from 0.96 above [Formula: see text] = 35.96 g kg-1 to 4.99 below [Formula: see text] (~-620%). Although salinity's significance in explaining SOC variability is minor (<6%), we estimate a one SD increase in salinity of topsoil samples (0 to 7 cm) correlates with respective [Formula: see text] declines of ~4.4% and ~9.26%, relative to [Formula: see text] and [Formula: see text]. The [Formula: see text] decline in croplands is greatest in vegetation/cropland mosaics while lands covered with evergreen needle-leaved trees are estimated with the highest [Formula: see text] decline in noncroplands. We identify soil nitrogen, land cover, and precipitation Seasonality Index as the most significant parameters in explaining the SOC's variability. The findings provide insights into SOC dynamics under increased soil salinity, improving understanding of SOC stock responses to land degradation and climate warming.

Effects of Short-Term Nitrogen Additions on Biomass and Soil Phytochemical Cycling in Alpine Grasslands of Tianshan, China

The nitrogen deposition process, as an important phenomenon of global climate change and an important link in the nitrogen cycle, has had serious and far-reaching impacts on grassland ecosystems. This study aimed to investigate the survival adaptation strategies of plants of different functional groups under nitrogen deposition, and the study identified the following outcomes of differences in biomass changes by conducting in situ simulated nitrogen deposition experiments while integrating plant nutrient contents and soil physicochemical properties: (1) nitrogen addition enhanced the aboveground biomass of grassland communities, in which Poaceae were significantly affected by nitrogen addition. Additionally, nitrogen addition significantly influenced plant total nitrogen and total phosphorus; (2) nitrogen addition improved the plant growth environment, alleviated plant nitrogen limitation, and promoted plant phosphorus uptake; and (3) there was variability in the biomass responses of different functional groups to nitrogen addition. The level of nitrogen addition was the primary factor affecting differences in biomass changes, while nitrogen addition frequency was an important factor affecting changes in plant community structure.

Forms of nitrogen inputs regulate the intensity of soil acidification

Soil acidification induced by reactive nitrogen (N) inputs can alter the structure and function of terrestrial ecosystems. Because different N-transformation processes contribute to the production and consumption of H+ , the magnitude of acidification likely depends on the relative amounts of organic N (ON) and inorganic N (IN) inputs. However, few studies have explicitly measured the effects of N composition on soil acidification. In this study, we first conducted a meta-analysis to test the effects of ON or IN inputs on soil acidification across 53 studies in grasslands. We then compared soil acidification across five different ON:IN ratios and two input rates based on long-term field N addition experiments. The meta-analysis showed that ON had weaker effects on soil acidification than IN when the N addition rate was above 20 g N m-2  year-1 . The field experiment confirmed the findings from meta-analysis: N addition with proportions of ON ≥ 20% caused less soil acidification, especially at a high input rate (30 g N m-2  year-1 ). Structural equation model analysis showed that this result was largely due to a relatively low rate of H+ production from ON as NH3 volatilization and uptake of ON and NH4 + by the dominant grass species Leymus chinensis (which are both lower net contributors to H+ production) result in less NH4 + available for nitrification (which is a higher net contributor to H+ production). These results indicate that the evaluation of soil acidification induced by N inputs should consider N forms and manipulations of relative composition of N inputs may provide an effective approach to alleviate the N-induced soil acidification.

Short-term but not long-term perennial mugwort cropping increases soil organic carbon in Northern China Plain

Perennial cropping has been an alternative land use type due to its widely accepted role in increasing soil carbon sequestration. However, how soil organic carbon (SOC) changes and its underlying mechanisms under different cropping years are still elusive. A chronosequence (0-, 3-, 6-, 20-year) of perennial mugwort cropping was chosen to explore the SOC dynamics and the underlying mechanisms in agricultural soils of Northern China Plain. The results revealed that SOC first increased and then decreased along the 20-year chronosequence. The similar patterns were also found in soil properties (including soil ammonium nitrogen, total nitrogen and phosphorus) and two C-degrading hydrolytic enzyme activities (i.e., α-glucosidase and β-glucosidase). The path analysis demonstrated that soil ammonium nitrogen, total nitrogen, and plant biomass affected SOC primarily through the indirect impacts on soil pH, total phosphorus availability, and C-degrading hydrolytic enzyme activities. In addition, the contributions of soil properties are greater than those of biotic factors (plant biomass) to changes in SOC across the four mugwort cropping years. Nevertheless, the biotic factors may play more important roles in regulating SOC than abiotic factors in the long run. Moreover, SOC reached its maximum and was equaled to that under the conventional rotation when cropping mugwort for 7.44 and 14.88 years, respectively, which has critical implications for sustainable C sequestration of agricultural soils in Northern China Plain. Our observations suggest that short-term but not long-term perennial mugwort cropping is an alternative practice benefiting soil C sequestration and achieving the Carbon Neutrality goal in China.