Abstract
Chemical fertiliser nitrogen addition will inhibit biological nitrogen fixation (BNF) for soybean (Glycine max [L.] Merr) growth. The optimal balance of these two nitrogen input sources has been a key issue for sustainable development in Northeast China. We used the data collected from a four-year experiment with varied irrigation and fertiliser treatments from 2007 to 2010 to evaluate the SPACSYS (Soil-Plant-Atmosphere Continuum SYStem) model. The validated model was run to investigate the responses to different management practices in seed yield, BNF, protein yield and soil nitrogen budgets. Scenario testing showed average yield increase of 2.4–5.2% with additional 50–100 kg N/ha application. Irrigation at the reproductive stage improved seed yield in drier years with an increase of 12–33% compared with the rain-fed treatment. BNF was suppressed by fertiliser nitrogen application and drought stress with a decrease of 6–33% and 8–34%, respectively. The average nitrogen budget without fertilization indicated a deficit of 39 kg N/ha. To attain higher seed yield, applying fertiliser at 25–30 and 15–20 kg N/ha before sowing is advised in drier and wetter years, respectively. To achieve a higher seed nitrogen content, an application rate of 55–60 and 45–50 kg N/ha is recommended for drier and wetter years, respectively.
Highlights
The soybean (Glycine max [L.] Merr) is the fourth largest crop in China and one of the main crops in the Northeast China plain (NEP)
The model algorithms are described in detail elsewhere [22,30,31], but we present here the specific calculations for the biological N2 fixation (BNF) rate (Ffix) based on root biomass: F f ix = fnodule × Wroot (106.6232 − 9.0213 × lgNcon) × ft × fw where Wroot is root biomass; fnodule is the fraction of nodule in root biomass; Ncon is soil mineral N content, which may inhibit the process of N2 fixation at higher values [27,32,33]; and ft and fw are the temperature and water impact factors, respectively:
The model captured the trend in leaf biomass and leaf area index (LAI) well, with R2 > 0.9 and EF > 0.8 (Figure 2B–F, Table 3)
Summary
The soybean (Glycine max [L.] Merr) is the fourth largest crop in China and one of the main crops in the Northeast China plain (NEP). BNF normally starts at the third node (V3, three nodes on the main stem with fully developed leaves beginning with the unifoliolate nodes) stage [5,6,7] with a lower rate until the sixth node (V6) stage [5,6,7] It reaches a maximum from pod setting to the beginning of the seed filling stage, which coincides with the peak N demand for protein synthesis and decreases [7,8,9,10]. Drought stress limits the above-ground biomass accumulation, and root exploration and nodule activity, resulting in a decrease in BNF rate and N uptake [9,11,12] If this occurs at the reproductive stage, yield depression will be more severe [9]
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