Effects Of Nitrogen And Phosphorus Addition On The Structure,Carbon Content And Stability Of Soil Aggregates In Songnen Grassland

With the increasing attention of the international community on climate change and greenhouse gas emission reduction,soil carbon sequestration has become one of the hotspots in global change and greenhouse effect research.Carbon sequestration has become one of the most important strategies to mitigate global climate change caused by an increase in CO₂ concentrations in the atmosphere.The physical protection of soil aggregates can extend the soil carbon turnover time and contribute to soil carbon sequestration.At present,research on soil aggregates is more concentrated on forest and farmland ecosystems.However,the research on grassland ecosystems is relatively scarce.The degree and mechanism of global change and land management and utilization on the distribution and stability of grassland soil aggregates are not yet clear.In this experiment,we used Songnen grassland as the research object,carried out four nitrogen gradients(0,5,10&20 g N m^-2 yr^-1)and two phosphorus gradients(0&5 g P m^-2 yr^-1)and their interactions for 4 years?2015-2018?.We used the wet sieving method to screen soil samples,and explored the response of the plant biomass,composition of soil aggregates,carbon content and microbial abundance of bulk soil and aggregates to nitrogen and phosphorus addition treatments.The primary findings are:?1?There were differences in the composition proportion of soil aggregates between different soil layers.For surface soil?0-10 cm?,the proportion of microaggregates was higher than the proportion of silt+clay fraction.For deep soil?10-30 cm?,the proportion of aggregates from high to low:microaggregates>silt+clay fraction>macroaggregates.Due to the greater disturbance intensity in the surface soil,the soil aggregates stability?mean weight diameter and geometric mean diameter?was less than that in the deep soil.With the increase of the nitrogen addition level,the proportion of large particle size aggregates?microaggregates?and the soil aggregates stability significantly increased,while the proportion of small particle size aggregates?silt+clay fraction?significantly decreased.Compared with the nitrogen addition treatment,the interaction of nitrogen and phosphorus increased the proportion of small particle size aggregates,and reduced the proportion of large particle size aggregates and the soil aggregates stability.The soil aggregate stability was significantly positively correlated with soil organic carbon content and microbial abundance.?2?The carbon contents?soil total carbon,soil organic carbon,dissolved organic carbon and microbial biomass carbon?were different between the studied soil layers.Due to the good hydrothermal and aeration conditions in the surface soil,and high accumulation of the plant surface litter,and the high soil microbial activity maked the soil carbon contents in the surface soil significantly higher than that in the deep soil.Due to the large specific surface area and strong adsorption capacity of small particle size aggregates,the carbon contents of silt+clay fraction were significantly higher than that of microaggregates.The nitrogen and phosphorus addition treatments significantly increased the organic carbon content and dissolved organic carbon content of the bulk soil and aggregates.With the increase of the level of nitrogen addition treatment,the microbial biomass carbon content of the bulk soil and aggregates significantly decreased.Compared with nitrogen addition treatment,nitrogen-phosphorus interaction treatment significantly increased microbial biomass carbon content.?3?The microbial abundance in the studied soil layers were significantly different between surface soil and deep soil..Because the organic matter of microbial substrates in the surface soil were significantly higher than that in the deep soil,the microbial abundance in the surface soil were significantly higher than that in the deep soil.Aggregate particle size mediates microbial colonization,silt+clay fraction were more microbial abundance than microaggregates because they contain more organic matter and have a larger ratio surface area.The nitrogen and phosphorus addition significantly increased bulk soil and soil aggregates microbial abundance.In summary,the nitrogen addition treatments significantly increased the organic carbon content and microbial abundance of the soil aggregates binder,thereby significantly increased the proportion of large particle size aggregates and the soil aggregates stability,while the result of phosphorus addition treatments was the opposite.The formation and soil aggregates stability provided physical protection for soil carbon,reduced microbial degradation and interference,and thus improved soil carbon sequestration capacity.The results of the study are helpful to assess the impact of nitrogen and phosphorus nutrient element addition on grassland management measures on soil carbon sequestration potential.