| Root residues in the field after harvesting crops are the basic materials for soil organic carbon(C), and they play important roles to maintain soil organic matter and improve soil fertility. The application of inorganic fertilizers not only increases crop yield, but also affects the allocation of photosynthate above-and below ground. The effect of different fertilization on the crop root biomass returned into soil has been studied widely. However, little research has focused on the changes of root chemical composition induced by different fertilization. The nitrogen(N) fertilizer application accounts for the largest amount in agricultural production. However, it was still not clear whether the chemical composition and decomposition dynamics of crop roots would be affected by N rates, in the meantime, soil nutrient cycle may also be affected by root decay. Consequently, the hypotheses in this study are those the N fertilizer application affects root biomass remained in soil, as well as the chemical composition of maize roots and their decomposition in soil, thereafter the C sequestration in soil profile. The difference in crop root decomposition induced by various hydrothermal conditions may affect soil organic C accumulation.Therefore, we have studied the effects of long-term different N fertilization rates on the chemical composition and decay dynamics of maize roots with different methods, including incubation methods, field experiments, and δ13C technique. Soil organic and inorganic C stocks have also been investigated under different cultivation and N rates. The main conclusions showed as follows:(1) Maize roots in the 0-20 cm soil depth after their harvest in October 2011 were collected from different fertilization treatments of two long-term experiments(21 and 8 years, respectively) located at the south edge of the Loess Plateau. The root biomass and nutrient contents were measured. The results indicated that compared with control(without any fertilizer)(CK), N, NK, and PK fertilizer application, the dry weight of maize roots increased significantly(P<0.05) in the treatments of NP, NPK, combined chemical fertilizer and manure(M1NPK, M2NPK), as well as chemical fertilizer with crop straw return(SNPK). And in contrast to the treatment without any N fertilizer addition(N0), 120(N120) and 240 kg N ha-1(N240) fertilizations obviously increased root dry weight by 38% and 45% respectively. The accumulative amounts of C, N, P, and K in maize roots were significantly higher in the NP, NPK, M1 NPK, M2 NPK, SNPK, N120, and N240 treatments compared to others, particularly in the combined chemical fertilizer and manure treatments The portion of water soluble organic carbon and total soluble nitrogen in maize roots were greater in the SNPK and N application treatments, respectively. Root cellulose and lignin contents were the lowest in the NP and M1 NPK treatments, respectively. Root C to N and lignin to N ratios were significantly higher in the CK, PK, and N0 treatments compared with others. We concluded that different fertilizers application and their rates affected both maize roots biomass and their chemical compositions. The balancing fertilization(e.g. NP, NPK, MNPK, and SNPK treatments) could increase the organic residues inputs and nutrient accumulations, which would be helpful for improving soil fertility and C sequestration.(2) An incubation experiment was conducted to study the organic C mineralization characteristics of maize roots derived from three N rates(0, 120, and 240 kg N ha-1; R0, R120, and R240), as well as the influences of root decomposition on soil available C and N mixed with soils(root: soil=2: 100) collected from 15 cm and 45 cm layers. It was found that compared to the maize root without N application(R0), the total N content of R120 and R240 increased significantly by 90%~104%, and the root C to N ratio decreased by 43%~50%(R0>R240>R120). After the 105-d incubation, the amount of cumulative CO2 release was significantly greater in R120 and R240 than that of the R0, however, there was no obvious difference between the R120 and R240. And there showed a significantly negative correlation between root C to N ratio and its cumulative CO2 release(P<0.01). The organic C mineralization ratio of R0, R120, and R240 was 33%, 40%, and 38% in the 15 cm soil, and was 22%, 32%, and 29% in the 45 cm soil, respectively. Significant differences were found in the root C mineralization ratio among R0, R120, and R240, and root C mineralization ratio in the 15 cm soil was higher by 26%~52% than that in the 45 cm soil. Soil microbial biomass C and soluble organic C contents increased significantly in the maize root addition compared with the control soil, and soil mineral N content increased at the beginning of incubation in the R120 and R240 addition treatments. These findings demonstrate that different N fertilization rates could not only affect the chemical compositions of maize root, but also its decomposition characteristics under incubation condition.(3) We determined the decomposition dynamics of different N rates fertilized maize roots(0, 120, and 240 kg N ha-1; R0, R120, and R240) in the depths of 15 and 45 cm in Lou soil(Yangling) for 386 days. The root dry weight was 64%~69% greater, and the lignin to N ratio was 51%~57% less in the R120 and R240 compared with R0. After 386-d decomposition, the portion of remaining root C in R0, R120, and R240 was 36.3%, 25.2%, and 28.7% in the 15 cm depth; and was 38.4%, 30.6%, and 31.1% in the 45 cm depth. Portion of root C remaining was positively correlated with its lignin content, C to N and lignin to N ratios, whereas it was negatively correlated with root total N content. Compared to the R0 addition soil samples, the contents of soil mineral N in the R120 and R240 addition treatments were greater by 23%~37%. And the microbial biomass C and soluble organic C contents in soils mixed with maize roots were greater by 143%~297% and 20%~118%, respectively, compared to the control soil treatment. Hence, long-term different N rates did affect the decomposition of crop roots in the field soil. The increased root N content and decreased C to N ratio with N fertilization resulted in slightly faster root C decomposition. Maize roots from N fertilized plots left more organic C in soil due to their much greater biomass; therefore, N fertilization would lead to a greater C input.(4) A field decomposition experiment was conducted to evaluate the organic C decay of three N rates fertilized maize roots(0, 120, and 240 kg N ha-1; R0, R120, and R240), as well as the dynamics of soil available C and N contents affected by roots addition in depths of 15 and 45 cm in black loessial soil(Changwu) for 368 days. In contrast to control soil, the soil microbial biomass C, soluble organic C, and mineral N contents were significantly increased in the maize roots addition treatments. The portion of remaining root C in R0, R120, and R240 was 44.4%, 35.3%, and 34.9% in the 15 cm depth; and was 53.3%, 44.3%, and 42.5% in the 45 cm depth after 1-yr decomposition. Carbon decomposition ratio and the decay rate constant of maize roots were both significantly greater in the 15 cm than that in the 45 cm soil depth. The time to reach 95% C decomposition of R0, R120, and R240 was 3.2, 2.3, and 1.9 years longer, respectively in the 45 cm compared to that in the 15 cm depth. The structure of soil soluble organic matter became more complex as root decomposition gradually. Therefore, the influence of N fertilization on the nutrient composition of crop root and the effect of root decomposition on the C and N cycle should be paid more attention to when we focus on the global C sequestration in agricultural soils.(5) We investigated the differences in decomposition of three N rates fertilized maize roots(R0, R120, and R240) in Lou soil(Yangling) and black loessial soil(Changwu), as well as the factors influencing root decay. The δ13C technique was used to determine the contribution of root C decomposition to SOC turnover. During root decomposition experiment period, the better hydrothermal condition was found in Yangling. The cumulative precipitation in Yangling was 215 mm much more than that in Changwu; and the average soil temperature was 3.2oC and 4.2oC greater, respectively in the 15 cm and 45 cm depths in Yangling. Amount of the remaining root C in the black loessial soil was 21%~44% higher than in the Lou soil after 1-yr decay. The root-derived SOC percent was 22.4%~44.7% and 27.4%~38.3% in the Lou soil and black loessial soil, respectively. The root-derived SOC content was significantly greater in black loessial soil(P<0.05), however, obvious difference was absent among R0, R120, and R240. We conclude that faster root decomposition occurs in Yangling under better hydrothermal conditions. Additionally, soil depth, texture, and fertilization management would also affect organic litter decomposition and soil organic C sequestration and turnover.(6) A long-term field experiment initiated in 2003 was carried out to study the effects of crop cultivation practices and N fertilization rates on the stocks of soil organic carbon(SOC) and inorganic carbon(SIC), as well as C stable isotope signature(δ13C). The four cultivation practices were 1) fallow(FA), 2) conventional cultivation(CC), 3) straw mulch(SM), and 4) plastic film-mulched ridge with straw-mulched furrows(RF), each of them at three N fertilizer rates(0, 120, and 240 kg N ha-1; N0, N120, and N240, respectively). The SOC pool accounted for 54%~59% of the total soil C in the 0-100 cm layer, and SIC accounted for 41%~46%. Compared with the 8-yr fallow and conventional cultivation, crop straw addition(SM and RF) strongly increased the contents of soil microbial biomass C and soluble organic C, and increased SOC stock by 10% in the upper 20 cm. SIC stock in the entire 0-100 cm depth significantly increased by 19% under crop cultivation(CC, SM, and RF) compared to the fallow, particularly in the upper 60 cm. Moderate N application(120 kg N ha-1) increased SOC stock in the upper 40 cm depth, whereas the SIC stock declined with the increasing N application by the reduction of soil p H. The δ13C values of SOC increased with depth, whereas SIC δ13C values decreased with depth. Crop cultivation(CC and SM) and excessive N addition(N240) resulted in lower δ13C values of SOC and SIC, respectively. We conclude that combined application of crop residues return and moderate N fertilizer could contribute to the soil organic and inorganic C sequestration, which may also affect soil δ13C values. |
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