Submitted to: Precision Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/16/2011
Publication Date: 9/16/2011
Publication URL: dx.doi.org/10.1007/s11119-011-9244-3
Citation: Dao, T.H., Cao, Q., Khosla, R., Cui, Z., Chen, X., Miao, Y. 2011. Quantifying spatial variability of indigenous nitrogen supply for precision nitrogen management in small scale farming. Precision Agriculture. 13(1):45-61.. Interpretive Summary: Understanding how variable the indigenous nitrogen (N) supply of the soil in a large field is important to the potential implementation of precision nutrient management strategies in such agricultural fields. The soil indigenous N supply in small-scale intensive farming of the North China Plain were studied using seven adjoining farmers’ fields receiving no fertilizer application to determine the variability and spatial structure of the soil nitrogen supplying capacity to winter wheat. This knowledge will allow application of a precision nitrogen management approach and achieve potential savings in fertilizer used in these high-input systems. The study results indicated that soil indigenous N supply varied significantly both within individual fields and across fields. Observed average optimum nitrogen rates showed that field-specific nitrogen management can be a practical management strategy in small scale intensive cropping systems in soil indigenous N supply. Compared with the typical farmer’s practice, site-specific nitrogen management has the potential to save 105 pounds N per acre, preventing under- or over-fertilization in these fields. A leaf chlorophyll meter and a GreenSeeker crop canopy sensor can be used to diagnose changing winter wheat nitrogen status and estimate crop nitrogen requirements during the growing season. However, such instrumentation may become of limited value for high input intensive cropping systems, due to optical saturation or non-responsiveness of these meters encountered at nitrogen fertilization rates greater than 110 lbs/a. Additional studies are needed to evaluate new active canopy sensors and vegetation indices for such systems.
Technical Abstract: Understanding spatial variability of indigenous nitrogen (N) supply (INS) is important to the implementation of precision nitrogen management strategies in intensive agricultural fields of North China Plain (NCP). The objectives of this study were to determine 1) winter wheat (Triticum aestivum L.) N status variability and estimate in-season winter wheat N requirements, using a chlorophyll meter (CM) and an active canopy crop sensor, 2) field-to-field and within-field variability in INS and their spatial structures, and 3) the potential savings in N fertilizers using precision N management technologies. Seven farmers' fields in Quzhou County, Hebei Province were selected for this study, and no fertilizers were applied to the wheat crop. Plant samples (1m2) were collected on 25 x 25 m grids at Feekes 6 stage and at physiological maturity in 2007 to measure aboveground biomass, N concentration and uptake. At Feekes 6 stage, CM readings and canopy normalized difference vegetation indices (NDVI) and ratio vegetation index (RVI) were collected using a SPAD 502 meter and GreenSeeker canopy sensors, respectively. Indigenous N supply varied significantly both within individual fields and across different fields, ranging from 33.4 to 319.5 kg ha-1, with an average of 143.8 kg ha-1 and a CV% of 34.7. The spatial dependence of INS, however, was low. Site-specific optimum N rates (SONR) varied from 0 to 357 kg ha-1 across the seven fields, with an average of 174 kg ha-1 and a CV of 46.1%. Field average optimum N rates indicated a good potential for field-specific N management, saving an average 127 kg N ha-1 compared to typical farmers' practices. Both CM readings (R2=0.59) and GreenSeeker spectral vegetation indices (NDVI, R2=0.58; RVI, R2=0.30) were significantly related to aboveground N uptake. While the wheat crop received no N fertilizer in this study, some saturation effects were observed in relationships between CM and NDVI readings and N uptake and SONRs, indicating the challenge of using such technologies for high input intensive cropping systems of NCP and the need to evaluate more suitable active canopy sensors and vegetation indices.