Effects Of Different Reduced Nitrogen Applications On Mineralization And Transformation Of Organic Carbon In Paddy Soils

The"win-win"of nitrogen fertilizer reduction and soil carbon sequestration is of great significance to the sustainable development of the paddy soil ecological environment in China.It could provide a theoretical basis for the possibility of reducing nitrogen fertilizer application and the effect of carbon sequestration in soil.Therefore,this study selected subtropical paddy soil with five different fertilization treatments to conduct a laboratory incubation for 100 d at 25^oC.The mineralization dynamics of organic substrates(¹³C-glucose)under different nitrogen reduction treatments and its effect on the native organic carbon mineralization of paddy soil was studied,and the decomposition and transformation of organic substrate and its distribution in different components of organic carbon（e.g.particulate organic carbon,microbial biomass carbon,dissolved organic carbon,calcium bonded organic carbon,and iron/aluminum bonded organic carbon）was explored in paddy soil.Fertilization treatments included no fertilizer（NF）,recommended nitrogen fertilizer（RF）,reduced nitrogen fertilizer（DF）,straw combined with low nitrogen fertilizer（RS+LF）,and cow manure combined with low nitrogen fertilizer（OM+LF）.The reduced nitrogen application rate is 75%of recommended nitrogen fertilizer,and the total nitrogen applied for RS+LF and OM+LF was the same to that for DF treatment.The purpose is to reveal the characteristics and differences of organic carbon turnover under different types of reduced nitrogen fertilization treatments in subtropical paddy soil.The main results are as follows:（1）During the 100-day cultivation period,the mineralization rates of organic substrate for NF,RF,DF,RS+LF and OM+LF treatments was 0.05～6.67 mg/（kg·d）,0.06～7.27 mg/（kg·d）,0.05～7.05 mg/（kg·d）,0.05～8.11 mg/（kg·d）and 0.05～7.41mg/（kg·d）,respectively;After 100 days of cultivation,the cumulative mineralization rate was 29%,28%,24%25%and 22%,separately,there is no significant difference in the cumulative mineralization rate between RF and DF,but RF is significantly higher than OM+LF.The above results indicate that reduced nitrogen fertilizer can reduce the accumulative mineralization rate of organic substrates in the paddy soil,and it is most significant to straw combined with low nitrogen fertilizer.The stimulating effect of organic substrates on the original organic carbon is shown as no fertilization treatment（42%）and recommended nitrogen fertilization（62%）are significantly higher than reduced nitrogen fertilizer treatment（18%）,and reduced nitrogen fertilizer treatment is significantly higher than straw combined with low nitrogen fertilizer（11%）and cow manure combined with low fertilizer（13%）.It can be seen that the original organic carbon of the soil under different fertilization treatments responds differently to the addition of exogenous organic substrates.The positive excitation effect of the reduced nitrogen fertilization treatment is significantly lower than that of recommended nitrogen fertilization.（2）The distribution ratio of organic substrate to activated carbon pool（a₁）is that straw and cow combined with low-quantity fertilizer（25.09%and 25.58%）are significantly higher than reduced nitrogen fertilizer（23.21%）,while there was no significant difference between the recommended nitrogen fertilization treatment（21.54%）and reduced nitrogen fertilizer treatment;The turnover rate showed that straw（0.301 mg/（kg·d））and cow manure combined with low nitrogen fertilizer（0.338mg/（kg·d））were significantly higher than the reduced nitrogen fertilizer（0.221mg/（kg·d））.There was no significant difference between the recommended nitrogen fertilization treatment and the reduced nitrogen fertilizer treatment（0.193 mg/（kg·d））,and the half-life showed the opposite trend.This shows that the low nitrogen fertilizer with straw and cow significantly increased the turnover rate of organic substrates in the soil,and shortened the turnover time of organic substrate mineralization in the soil compared with reduced-quantity fertilizer and recommended nitrogen fertilization.（3）The distribution ratio of decomposition and conversion products of organic substrates in paddy soil to organic carbon（SOC）,particulate organic carbon（POC）,microbial biomass carbon（MBC）and soluble organic carbon（DOC）is 54%～62%,4.8%～7.9%,5.4%～11.5%and 0.2%～0.4%.Among them,the distribution ratio of¹³C-SOC between DF（55%）and RF（54%）is not significant,but the distribution ratio between RS+LF（60%）and OM+LF（62%）is significantly higher than DF;The distribution ratio of ¹³C-POC between DF（6.1%）and RF（5.7%）is not significant,but the distribution ratio between RS+LF（6.8%）and OM+LF（7.9%）is significantly higher than that of DF;¹³C-POC The distribution ratio of MBC between DF（7.7%）and RF（6.6%）is not significant,but the distribution ratio between RS+LF（11.4%）and OM+LF（11.5%）is significantly higher than that of DF;¹³C-DOC is The distribution ratio between DF（0.3%）and RF（0.2%）was not significant,but the distribution ratio in RS+LF（0.5%）and OM+LF（0.4%）was significantly higher than DF.Therefore,there is no significant difference in the distribution ratio of organic substrate decomposition products in SOC components between recommended nitrogen fertilizer treatment and reduced nitrogen fertilizer treatment,but the distribution ratio in straw and cow manure combined with low nitrogen fertilizer treatment is significantly higher than that of reduced nitrogen fertilizer.（4）The distribution ratio of decomposition and transformation products of organic substrates in paddy soil to calcium-bonded organic carbon（Ca-SOC）and iron-aluminum bonded organic carbon（Fe/Al-SOC）is 0.05%～0.14%,0.37%～0.94%.The distribution ratio of ¹³C-Ca/SOC between DF（0.07%）and RF（0.06%）is not significant,but the distribution ratio between RS+LF（0.14%）and OM+LF（0.13%）is significantly higher than DF;The distribution ratio of ¹³C-Fe/Al/SOC between DF（0.65%）and RF（0.69%）is not significant,but the distribution ratio among RS+LF（0.89%）and OM+LF（0.94%）is significantly higher DF.It can be seen that,compared with recommended nitrogen fertilizer,even under the premise of reduced nitrogen fertilizer,organic and inorganic fertilizers in proportion to the application of organic matter in the soil still have stronger chemical protection of mineral components and are more conducive to the accumulation and stability of soil organic carbon.