Exploring site-specific N application rate to reduce N footprint and increase crop production for green manure-rice rotation system in southern China.

Milk vetch (Astragalus sinicus L.) is leguminous green manure (GM) which produces organic nitrogen (N) for subsequent crops and is widely planted and utilized to simultaneously reduce the use of synthetic N fertilizer and its environmental costs in rice systems. Determination of an optimal N application rate specific to the GM-rice system is challenging because of the large temporal and spatial variations in soil, climate, and field management conditions. To solve this problem, we developed a framework to explore the site-specific N application rate for the low-N footprint rice production system in southern China based on multi-site field experiments, farmer field survey, and process-based model (WHCNS_Rice, soil water heat carbon nitrogen simulator for rice). The results showed that a process-based model can explain >83.3% (p < 0.01) of the variation in rice yield, aboveground biomass, crop N uptake, and soil mineral N. Based on the scenario analysis of the tested WHCNS_Rice model, the simple regression equation was developed to implement site-specific N application rates that considered variations in GM biomass, soil, and climatic conditions. Simulation evaluation on nine provinces in southern China showed that the site-specific N application rate reduced regional synthetic N fertilizer input by 29.6 ± 17.8% and 65.3 ± 23.0% for single and early rice, respectively; decreased their total N footprints (NFs) by 23.4% and 49.3%, respectively; and without reduction in rice yield, compared with traditional farming N practices. The reduction in total NF was attributed to the reduced emissions from ammonia volatilization by 35.2%, N leaching by 28.4%, and N runoff by 32.7%. In this study, we suggested a low NF rice production system that can be obtained by combining GM with site-specific N application rate in southern China.

Effects of different nitrogen supply levels on the yield of Orychophragmus violaceus, soil residual inorganic nitrogen, and nitrogen balance.

Understanding the responses of winter green manure February orchid (Orychophragmus violaceus) to different levels of nitrogen (N) supply in Northern China and determining the optimal soil N supply level to meet N demands of green manure production with high yield and efficiency, could provide a theoretical foundation and practice reference for maximizing ecological effects of green manure and optimizing N management for spring maize-green manure rotation system in intensive farmland in Northern China. We carried out a field experiment in a site which had received no fertilizer for many years. The aboveground biomass accumulation, N uptake of February orchid and soil residual inorganic N before green manure incorporation, as well as the N balance during the green manure growing season were determined under different levels of N supply. The results showed that N fertilizer application significantly increased the biomass and N uptake of February orchid under low soil inorganic N content (15 kg·hm-2 in 0-90 cm soil layer). At the application rate of 90 kg·hm-2, the biomass (dry mass) and N uptake reached the maximum, being 2031 and 42 kg·hm-2, respectively. The soil residual inorganic N amount rose with the increases of N fertilizer application before sowing, growing very rapidly once the application rate was over 60 kg·hm-2. With the increases of N application rate, the calculated apparent N balance changed from deficit to surplus in the growing season of February orchid. The inputs and outputs of N reached a balance at the application rates of 60 to 90 kg·hm-2. The relationships between February orchid biomass, N uptake, soil inorganic N before green manure incorporation, and soil N supply amount (0-90 cm preplant soil inorganic N content plus N application rate) could be fitted by the quadratic, linear plus plateau and exponential models respectively. Based on the simulation, we calculated the preplant soil N supply and soil residual inorganic N content before green manure incorporation would be 136 and 78 kg·hm-2 individually, as the biomass of February orchid reached the maximum (2010 kg·hm-2). While N uptake was at the highest level of 40 kg·hm-2, the biomass of February orchid was 95% of the maximum biomass mentioned above (1919 kg·hm-2) and the soil residual inorganic N before green manure incorporation decreased to 57 kg·hm-2 whose corresponding minimum soil N supply amount was 105 kg·hm-2. This value was quite near to the recommended soil residual inorganic N (100 kg·hm-2) after maize harvest under optimized N management in Nor-thern China. Taken together, our results showed that the level of soil N supply should be at approximately 100 to 105 kg·hm-2 in spring maize-winter green manure system for improving tradeoffs between agronomic and environmental impacts.

[Effects of Green Manures on Soil Dissolved Organic Matter in Moisture Soil in North China].

Soil dissolved organic matter (DOM) plays an important role in the biogeochemistry of carbon, nitrogen, and phosphorus and in the transport of heavy metals and pesticides in soil. In moisture soil, green manures and soils were sampled in situ at the ploughed stage of green manures. A 56-day laboratory incubation experiment was conducted to simulate the dynamic changes of soil DOM influenced by the decomposition of green manures, the green manures were Hairy vetch (Vicia villosa Roth.), February orchid (Orychophragmus Violaceus L.), Rye (Secale cereale L.), the soil without green manure was used as a control (CK). The composition and ultraviolet-visible spectrum parameters of soil DOM were investigated at different incubation stages. Results showed that green manures could increase the dissolved organic carbon (DOC), the total organic acids (TOAs) and total carbohydrate (TCs) contents, and all treatments were reached a peak on the 1st day and decreased later. Hairy vetch affected DOC and TOAs most and were increased by 114.01% and 109.10% higher than CK respectively at the 1st and 14th day. Rye influenced the total carbohydrate (TCs) most and was maximumly 323.18% higher than CK at the 42nd day. Green manures could increase the dissolved organic nitrogen (DON) content, DON in all green manure treatments increased on the 1st day, decreased several days later and increased again after 20～30 d. Hairy vetch effected DON best and was 305.83% higher than CK at the 42nd day. All green manures increased in SUVA254, SUVA260, SUVA272, SUVA280 and SAUC240-400, while decreasing in A250/A365 and A240/A420. The PCA analysis of ultraviolet-visible spectrum parameters showed that SUVA254, SUVA260, SUVA272, SUVA280 had a high positive correlation between each other, and the same situation was found between A250/A365 and A240/A420. Among them, SAUC240-400 was a key factor parameter featuring the characteristics of DOM. The results suggested that green manures could increase the contents of DOM and its aromaticity, hydrophobic percentage, humification degree and average molecular weight, and could be increased and the stability of DOM could be enhanced accordingly. The ultraviolet-visible spectrum parameters could indicate the changes of characteristics of DOM in this study.

[Long-Term Different Fertilizations Changed the Chemical and Spectrum Characteristics of DOM of the Irrigation-Desert Soil in North-Western China].

By using Ultraviolet-visible Spectrometry, Fourier Transform Infrared Spectrometer and Elemental Analyzer, spectrum and chemical characteristics of soil DOM affected by long-term different fertilizations were investigated in irrigation-desert soil in North-western China based on an experiment started from 1988. Four different fertilization treatments were included, i. e., organic fertilizer (OF), green manure (GM), chemical fertilizer (CF) and a control of no fertilization (CK). The results showed that fertilization could increase the contents of DOM. Compared to CK, the treatments of OF, GM, CF increased the dissolved organic carbon (DOC) by 37%, 29%, 16%; increased the dissolved nitrogen (DON) by 334%, 257%, 182%; increased the total carbohydrate (TCs) by 90%, 25%, 2%; and increased the total organic acids (TOAs) by 195%, 116%, 58%; respectively. Furthermore, DOC, DON, TCs, and TOAs in the OF treatment were significantly higher than those in CK, they were also significantly higher in the GM and CF treatments except for TCs. The ultraviolet-visible analysis showed that fertilizations enhanced the SUVA(254), SUVA(260), SUVA(272) and SUVA(280) of DOM, indicating that fertilizations increased the aromatic and hydrophobic percentage, humification degree, and average molecular weight, and thus resulting in more stability of DOM. Same trends were showed for all the 4 ultraviolet spectrum absorption values in different fertilizations, i. e., the strongest effect was found in the OF treatment, and then was the GM treatment and CF treatment successively. From the results by the Fourier Transform Infrared Spectrometry, the characteristic peak of aromatic in the OF treatment was observed shifting from 1 625 to 1 649 cm(-1), which was close to the characteristic peak of humin, suggesting that the aromaticity of DOM in the OF treatment was higher than the other treatments. The characteristic peaks of C-O at 1 260-1 000 cm(-1) belonging to sugar, alcohol, and carboxylic acid were highest in the GM treatment, showing that the green manure could increase rich oxygen radicals. The highest characteristic peaks of N-H at 3 559, 3 419 and 1 456 cm(-1) were observed in the CF treatment, indicating that the chemical fertilizer could increase amine substances. The contents of C, O and N in the OF, GM, CF treatments were also increased respectively according to the elemental analysis.

[Temporal and spatial variations of soil NO(3-)-N in Orychophragmus violaceus/spring maize rotation system in North China].

The February orchid (Orychophragmus violaceus)-spring maize rotation system is established to resolve the problems caused by the expansion of fallow fields in North China. Based on a site-specific experiment, temporal and spatial variations of soil NO(3-)-N were investigated during the period from February orchid incorporation to maize harvest. The results showed that the nitrate content in soil profiles not only showed a temporal characteristic, i. e., increasing at the beginning of the maize season and decreasing then after, but also showed a spatial characteristic, i. e., the gradual occurrence of the peak of nitrate content from shallower to deeper layer with the growth season of maize. Meanwhile, incorporation of February orchid could affect temporal and spatial variations of soil NO(3-)-N. February orchid planting reduced the soil NO(3-)-N accumulation in the profile of 0180 cm. After incorporation of February orchid, similar characteristics were observed at the seedling and bell stages of maize, i. e., the soil NO(3-)-N mainly stayed in the profile of 0-20 cm, and NO(3-)-N concentrations in the treatments with February orchid were higher in 0-100 cm layer and lower in 100-180 cm layer than those of the treatments without February orchid. After tasseling stage, opposite phenomena were found, and the soil NO(3-)-N content was all relative low. Overall, incorporation of February orchid could increase the storage capacity of soil NO(3-)-N in the profile of 0-180 cm.