Nitrogen Mineralization from Weed Residues

View the project final report

• 2011 annual report

Commodities

• Agronomic: corn

Practices

• Crop Production: nutrient cycling
• Pest Management: weed ecology
• Production Systems: general crop production
• Soil Management: nutrient mineralization, organic matter, soil analysis, soil quality/health

Weed residue may significantly contribute to nitrogen (N) cycling in agro-ecosystems. Identifying this potential N sink or source is essential for maximizing corn grain yield. A laboratory experiment was designed to examine the effect of N application rate and weed removal height on the chemical composition and N mineralization from weed residue. Common lambsquarters, common ragweed, and giant foxtail were grown with varying N application rates in the field and collected when they were either 10 or 20 cm tall. In a laboratory incubation study, dried, ground weed residue was mixed with field moist soil and N mineralization was measured over a 12 wk period by determining the inorganic N content of the soil. The carbon:N (C:N) ratio was greatest for giant foxtail when grown with 0 kg N/ha. Weeds that were 20 cm tall had a greater C:N ratio than 10 cm tall weeds. The C:N ratio of weed residue from all treatments decreased with increasing N application rate. Nitrogen was immobilized by giant foxtail grown with 0 kg N/ha from 1 to 8 wk of incubation. Nitrogen was immobilized by 20 cm tall weeds from 1 to 2 wk of incubation. For the other treatment combinations, N mineralization was generally rapid up to 4 wk of incubation. After 4 wk of incubation, N mineralization plateaued. At 12 wk of incubation, 13 and 19% of the total N in weed residue was mineralized by giant foxtail grown with 0 kg N/ha and 20 cm tall weeds grown with 0 kg N/ha, respectively. For all treatment combinations, 32 to 60% of the total N in weed residue was mineralized by 12 wk of incubation. The rate of N mineralization was negatively correlated with C:N ratio and positively correlated with extractable nitrate-N (NO3-N) of the weed residue. Quantity of potentially mineralizable N was similar among treatments and was not influenced by weed residue chemical composition. Weed residue may significantly contribute to the soil N pool; however, N mineralization is influenced by the chemical composition of the weed residue while the chemical composition of weed residue varies with N application rate, weed height, and weed species.

Introduction:

Understanding nutrient cycling in agro-ecosystems and identifying potential nitrogen (N) sinks and sources is essential for maximizing corn grain yield while minimizing environmental impact. To minimize environmental impact, N availability needs to correspond to rapid N assimilation by corn. To avoid corn grain yield reductions, weed control is recommended in the North Central US prior to the V4 to V6 corn growth stage (Gower et al. 2003; Dalley et al. 2006). Nitrogen mineralization of weed residues subsequent to weed control may contribute to soil N pools. Net N mineralization of plant residues occurs at C:N ratios less than 30. It is well documented that weeds assimilate large quantities of N (Chaves et al. 2004; De Neve and Hofman 1996), but little is understood about the fate of weed residue subsequent to postemergence weed control. Nitrogen mineralization from weed residues may contribute to soil N pools just prior to corn N demands; however, N mineralization later in the growing season may result in N losses to the environment.

Our objectives were to 1.) evaluate the effect of weed species, size, and N application rate on the extractable NO3-N concentration and C:N ratio of weed residue, 2.) determine the quantity and rate of N mineralization from these weed residues, and 3.) examine the correlation between nitrate-N (NO3-N) concentration and C:N ratio and quantity and rate of N mineralization of weed residue.