Short-term phosphorus addition augments the effects of nitrogen addition on soil respiration in a typical steppe

Soil ecoenzyme activities coupled with soil properties and plant biomass strongly influence the variation in soil organic carbon components in semi-arid degraded wetlands

Wetland degradation can induce alterations in plant biomass, soil properties, and soil ecoenzyme activities, consequently influencing soil organic carbon components. Despite extensive investigations into the relationships among plant characteristics, soil properties, and soil organic carbon components, the enzymatic mechanisms underlying changes in soil organic carbon components, particularly the impact and contribution of ecoenzyme activities, remain poorly understood. This study compared the soil organic carbon components at a depth of 0-20 cm in wetlands in the semi-arid western Songnen Plain under different degradation levels and explored plant biomass, soil properties, and soil ecoenzyme activities. The results showed that the soil total organic carbon, labile organic carbon, and recalcitrant organic carbon contents in the degraded wetlands were generally lower than those in the non-degraded wetlands. Furthermore, the soil nutrient contents and soil β-1,4-glucosidase, L-leucine aminopeptidase, and acid phosphatase activities were also lower in the degraded wetlands than in the non-degraded wetlands. Vector analysis of enzymatic stoichiometry revealed that wetland degradation did not increase microbial carbon limitation. The soil organic carbon components showed significant positive correlations with plant biomass, soil water content, soil total nitrogen, soil total phosphorus, as well as soil ecoenzyme activities. Variation partitioning analysis revealed that plant biomass, soil properties, soil ecoenzyme activities collectively accounted for 78.5 % variation in soil organic carbon components, among which plant biomass, soil properties, soil ecoenzyme activities, and their interactions explaining 4.2 %, 8.0 %, 7.9 %, and 24.5 % of the variation, respectively. Therefore, the impact of soil ecoenzyme activities and soil properties on soil organic carbon component changes was greater than that of plant biomass, with the interaction of these three factors playing a crucial role in soil organic carbon formation. This study provides a theoretical basis for scientifically evaluating the carbon sink function of degraded wetland soil and preserving the wetland soil carbon pool.

Enhanced CO2 uptake is marginally offset by altered fluxes of non-CO2 greenhouse gases in global forests and grasslands under N deposition

Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO2 exchange and the emission or uptake of non-CO2 GHGs (CH4 and N2 O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom-up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH4 and N2 O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (RH ) in both forests and grasslands, while a significant N-induced increase in soil CO2 fluxes (RS , soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N2 O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH4 , suggesting a declined sink capacity of forests and grasslands for atmospheric CH4 under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production-based method (forest, 1.28 Pg CO2 -eq year-1 vs. grassland, 0.58 Pg CO2 -eq year-1 ) was greater than those estimated using the SOC-based method (forest, 0.32 Pg CO2 -eq year-1 vs. grassland, 0.18 Pg CO2 -eq year-1 ) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%-4.8%) by the combined effects of its stimulation of N2 O emissions together with the reduced soil uptake of CH4 .

Clipping decreases plant cover, litter mass, and water infiltration rate in soil across six plant community sites in a semiarid grassland

Water infiltration in the soil is a crucial hydrological function in the land water cycle, especially in the semiarid region where water is relatively scarce. The semiarid grassland in Northern China represents the regional vegetation in the vast area of Eurasian continent and is sensitive to land use change. However, no clear patterns exist regarding the comprehensive examination of water infiltration in relation to clipping across six plant community sites. This study aimed to test the effect of clipping and plant community sites, which were dominated by Agropyron cristatum, Stipa krylovii, Leymus chinensis, Potentilla tanacetifolia, Artemisia frigida, or Lespedeza davurica, on the water infiltration rate in the semiarid grassland. Clipping significantly decreased the water initial, steady, and average infiltration rates by 39.13, 4.36, and 12.46 mm h^-1, respectively, across the six plant community sites. Clipping-induced changes in the average infiltration rate positively correlated with the changes in the plant cover (r = 0.60, P < 0.01), litter mass (r = 0.53, P < 0.01), forb functional group ratio (r = 0.46, P = 0.03), and total porosity (r = 0.49, P = 0.02), and negatively with water-holding capacity (r = -0.45, P = 0.03). Further, the water infiltration rate significantly differed among the six plant community sites. The L.davurica site had the highest water initial infiltration rate with a value of 137.63 ± 17.76 mm h^-1, while the L. chinensis site had the lowest rate with a value of 74.08 ± 5.26 mm h^-1. Principal component analysis showed that the total porosity, litter mass, plant cover, and forb functional group ratio were the main factors affecting water infiltration rates in the control grassland. Overall, our findings suggested that local governments and herders should implement unclipping as a potential sustainable management for improving hydrological function in the semiarid grassland.

Seasonal and diurnal soil respiration dynamics under different land management practices in the sub-tropical highland agroecology of Ethiopia

The temporal dynamics of soil respiration change in response to different land management practices are not well documented. This study investigated the effects of soil bunds on the monthly and diurnal dynamics of soil respiration rates in the highlands of the Upper Blue Nile basin in Ethiopia. Six plots (with and without soil bunds, three replicates) were used for measurement of seasonal soil respiration, and 18 plots were used for measurement of diurnal soil respiration. We collected seasonal variation data on a monthly basis from September 2020 to August 2021. Diurnal soil respiration data were collected four times daily (5 a.m., 11 a.m., 5 p.m., and 11 p.m.) for 2 weeks from 16 to 29 September 2021. A Wilcoxon signed-rank test showed that seasonal soil respiration rates differed significantly (p < 0.05) between soil bund and control plots in all seasons. In plots with soil bunds, seasonal soil respiration rates were lowest in February (1.89 ± 0.3 µmol CO₂ m^-2 s^-1, mean ± SE) and highest in October (14.54 ± 0.5 µmol CO₂ m^-2 s^-1). The diurnal soil respiration rate was significantly (p < 0.05) higher at 11 a.m. than at other times, and was lowest at 5 a.m. Seasonal variation in soil respiration was influenced by soil temperature negatively and moisture positively. Diurnal soil respiration was significantly affected by soil temperature but not by soil moisture. Further study is required to explore how differences in soil microorganisms between different land management practices affect soil respiration rates.

Soil Respiration of Paddy Soils Were Stimulated by Semiconductor Minerals

Large quantities of semiconductor minerals on soil surfaces have a sensitive photoelectric response. These semiconductor minerals generate photo-electrons and photo-hole pairs that can stimulate soil oxidation-reduction reactions when exposed to sunlight. We speculated that the photocatalysis of semiconductor minerals would affect soil carbon cycles. As the main component of the carbon cycle, soil respiration from paddy soil is often ignored. Five rice cropping areas in China were chosen for soil sampling. Semiconductor minerals were measured, and three main semiconductor minerals including hematile, rutile, and manganosite were identified in the paddy soils. The identified semiconductor minerals consisted of iron, manganese, and titanium oxides. Content of Fe₂O₃, TiO₂, and MnO in the sampled soil was between 4.21-14%, 0.91-2.72%, and 0.02-0.22%, respectively. Most abundant semiconductor mineral was found in the DBDJ rice cropping area in Jilin province, with the highest content of Fe₂O₃ of 14%. Soils from the five main rice cropping areas were also identified as having strong photoelectric response characteristics. The highest photoelectric response was found in the DBDJ rice cropping area in Jilin province with a maximum photocurrent density of 0.48 μA/cm². Soil respiration was monitored under both dark and light (3,000 lux light density) conditions. Soil respiration rates in the five regions were (from highest to lowest): DBDJ > XNDJ > XBDJ > HZSJ > HNSJ. Soil respiration was positively correlated with semiconductor mineral content, and soil respiration was higher under the light treatment than the dark treatment in every rice cropping area. This result suggested that soil respiration was stimulated by semiconductor mineral photocatalysis. This analysis provided indirect evidence of the effect semiconductor mineral photocatalysis has on the carbon cycle within paddy soils, while exploring carbon conversion mechanisms that could provide a new perspective on the soil carbon cycle.

Resistance and Resilience of Nine Plant Species to Drought in Inner Mongolia Temperate Grasslands of Northern China

Drought has been approved to affect the process of terrestrial ecosystems from different organizational levels, including individual, community, and ecosystem levels; however, which traits play the dominant role in the resistance of plant to drought is still unclear. The experiment was conducted in semi-arid temperate grassland and included six paired control and drought experimental plots. The drought treatment was completely removed from precipitation treatments from 20 June to 30 August 2013. At the end of the growing season in 2013, we removed the rain cover for ecosystem recovery in 2014. The results demonstrated that drought treatment increased the coverage of and abundance Heteropappus altaicus, Potentilla bifurca, and Artemisia scoparia by 126.2–170.0% and 63.4–98.9%, but decreased that of Artemisia frigida, Dontostemon dentatus, and Melissilus ruthenicu by 46.2–60.2% and 49.6–60.1%. No differences in coverage and abundance of Agropyron cristatum, Stipa kiylovii, and Cleistogenes squarrosa were found between control and drought treatment. The coverage and abundance of Stipa kiylovii have exceeded the original level before the drought stress, but Heteropappus altaicus still had not recovered in the first year after the disturbance. Our findings indicate that plant functional traits are important for the understanding of the resistance and resilience of plants to drought stress, which can provide data support for grassland management.

Contrasting Effects of Nitrogen Addition on Vegetative Phenology in Dry and Wet Years in a Temperate Steppe on the Mongolian Plateau

Changes in spring and autumn phenology and thus growing season length (GSL) pose great challenges in accurately predicting terrestrial primary productivity. However, how spring and autumn phenology in response to land-use change and nitrogen deposition and underlying mechanisms remain unclear. This study was conducted to explore the GSL and its components [i.e., the beginning of growing season and ending of growing season (EGS)] in response to mowing and nitrogen addition in a temperate steppe on the Mongolia Plateau during 2 years with hydrologically contrasting condition [dry (2014) vs. wet (2015)]. Our results demonstrated that mowing advanced the BGS only by 3.83 days, while nitrogen addition advanced and delayed the BGS and EGS by 2.85 and 3.31 days, respectively, and thus prolonged the GSL by 6.16 days across the two growing seasons from 2014 to 2015. When analyzed by each year, nitrogen addition lengthened the GSL in the dry year (2014), whereas it shortened the GSL in the wet year (2015). Further analyses revealed that the contrasting impacts of nitrogen on the GSL were attributed to monthly precipitation regimes and plant growth rate indicated by the maximum of normalized difference vegetation index (NDV_max). Moreover, changes in the GSL and its two components had divergent impacts on community productivity. The findings highlight the critical role of precipitation regimes in regulating the responses of spring and autumn phenology to nutrient enrichment and suggest that the relationships of ecosystem productivity with spring and autumn phenology largely depend on interannual precipitation fluctuations under future increased nitrogen deposition scenarios.

Differentiate Responses of Soil Microbial Community and Enzyme Activities to Nitrogen and Phosphorus Addition Rates in an Alpine Meadow

Nitrogen (N) and phosphorus (P) are the dominant limiting nutrients in alpine meadows, but it is relatively unclear how they affect the soil microbial community and whether their effects are rate dependent. Here, N and P addition rates (0, 10, 20, and 30 g m^-2 year^-1) were evaluated in an alpine meadow and variables related to plants and soils were measured to determine the processes affecting soil microbial community and enzyme activities. Our results showed that soil microbial biomass, including bacteria, fungi, gramme-negative bacteria, and actinomycetes, decreased along with N addition rates, but they first decreased at low P addition rates (10 g m^-2 year^-1) and then significantly increased at high P addition rates (30 g m^-2 year^-1). Both the N and P addition stimulated soil invertase activity, while urease and phosphatase activities were inhibited at low N addition rate and then increased at high N addition rate. P addition generally inhibited peroxidase and urease activities, but increased phosphatase activity. N addition decreased soil pH and, thus, inhibited soil microbial microorganisms, while P addition effects were unimodal with addition rates, achieved through altering sedge, and available P in the soil. In conclusion, our studies indicated that soil microbial communities and enzyme activities are sensitive to short-term N and P addition and are also significantly influenced by their addition rates.

Effects of short-term nitrogen and phosphorus addition on leaf stoichiometry of a dominant alpine grass

The effects of increasing nitrogen (N) and phosphorus (P) deposition on the nutrient stoichiometry of soil and plant are gaining improving recognition. However, whether and how the responses of N cycle coupled with P of the soil–plant system to external N and P deposition in alpine grassland is still unclear. A short-term external N and P addition experiment was conducted in an alpine grazing grassland in the KunLun Mountain to explore the effects of short-term N and P addition on the nutrient stoichiometry in soil and plant. Different rates of N addition (ranging from 0.5 g N m⁻² yr⁻¹ to 24 g N m⁻² yr⁻¹) and P addition (ranging from 0.05 g N m⁻² yr⁻¹ to 3.2 g P m⁻² yr⁻¹) were supplied, and the soil available N, P, leaf N and P stoichiometry of Seriphidium rhodanthum which dominant in the alpine ecosystem were measured. Results showed that N addition increased soil inorganic N, leaf C, leaf N, and leaf N:P ratio but decreased soil available P and leaf C:P. Furthermore, P addition increased soil available P, leaf P, soil inorganic N, leaf N, and leaf C and reduced leaf C:N, C:P, and N:P ratios. Leaf N:P was positively related to N addition gradient. Leaf C:P and leaf N:P were significantly negatively related to P addition gradient. Although external N and P addition changed the value of leaf N:P, the ratio was always lower than 16 in all treatments. The influences of P addition on soil and plant mainly caused the increase in soil available P concentration. In addition, the N and P cycles in the soil–plant system were tightly coupled in P addition but decoupled in N addition condition. The nutrient stoichiometry of soil and leaf responded differently to continuous N and P addition gradients. These data suggested that the alpine grazing grassland was limited by P rather than N due to long-term N deposition and uniform fertilization. Moreover, increasing P addition alleviated P limitation. Therefore, the imbalanced N and P input could change the strategy of nutrient use of the grass and then change the rates of nutrient cycling in the alpine grassland ecosystem in the future.

Nitrogen and phosphorus addition differentially enhance seed production of dominant species in a temperate steppe

Previous studies have demonstrated changes in plant growth and reproduction in response to nutrient availability, but responses of plant growth and reproduction to multiple levels of nutrient enrichment remain unclear. In this study, a factorial field experiment was performed with manipulation of nitrogen (N) and phosphorus (P) availability to examine seed production of the dominant species, Stipa krylovii, in response to N and P addition in a temperate steppe. There were three levels of N and P addition in this experiment, including no N addition (0 g N m-2 year-1), low N addition (10 g N m-2 year-1), and high N addition (40 g N m-2 year-1) for N addition treatment, and no P addition (0 g P m-2 year-1), low P addition (5 g P m-2 year-1), and high P addition (10 g P m-2 year-1) for P addition treatment. Low N addition enhanced seed production by 814%, 1371%, and 1321% under ambient, low, and high P addition levels, respectively. High N addition increased seed production by 2136%, 3560%, and 3550% under ambient, low, and high P addition levels, respectively. However, P addition did not affect seed production in the absence of N addition, but enhanced it under N addition. N addition enhanced seed production mainly by increasing the tiller number and inflorescence abundance per plant, whereas P addition stimulated it by decreasing the plant density yet stimulating height of plants and their seed number per inflorescence. Our results indicate seed production is not limited by P availability but rather by N availability in the temperate steppe, whereas seed production will be increased by P addition when N availability is improved. These findings enable a better understanding of plant reproduction dynamics in the temperate steppe under intensified nutrient enrichment and can inform their improved management in the future.