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.
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.
Weed residues were collected from a field study established at the Michigan State University Crop and Soil Sciences Research Farm in East Lansing in 2011. The field study was a randomized complete block design consisting of four N preplant application rates (0, 67, 134, and 202 kg N/ha) of incorporated urea and two weed heights (10 and 20 cm). There were three replications of treatments. Mixed, natural populations of weeds were allowed to emerge with the corn. Weed species primarily consisted of common lambsquarters, common ragweed, and giant foxtail. Above and below-ground portions of each weed species was collected when the average weed canopy height was 10 and 20 cm. Dried weed residue was finely ground. Total N and C content of the weed residues were measured. Extractable nitrate-N (NO3-N) from the weed residues was determined.
Mineralization from weed residue was evaluated at six incubation times (0, 1, 2, 4, 8, and 12 wk). There were two runs of the laboratory incubation experiment. Twenty grams dry weight equivalent moist soil was placed in specimen cups. The ground weed residue from each field treatment was mixed with the soil at a rate of 60 mg total N/kg soil. Controls for each incubation time consisted of soil without residue addition to account for N mineralization from soil organic matter. The containers were stored in the dark at 25 degrees C, and at each incubation time interval, soil was analyzed for ammonium-N (NH4-N) and NO3-N content.
Nitrogen mineralization from both soil organic matter and weed residue was considered to be the sum of NH4-N and NO3-N. Background N mineralization from soil organic matter was determined for each incubation time from the control (soil with no plant residue added) and subtracted from the treatments.
Extractable NO3-N was 4,865 and 5,419 mg NO3-N/kg for common ragweed grown with134 and 202 kg N/ha, respectively. Giant foxtail grown with 0 kg N/ha had the lowest extractable NO3-N content (94 mg NO3-N/kg). When grown with 0 kg N/ha, there were no differences in NO3-N among the three weed species; however, when weeds were grown with 67 to 202 kg N/ha, extractable NO3-N was greatest for common ragweed. Nitrate-N increased with N application rate for all weed species. Extractable NO3-N of weed residue was also significant by the interaction of weed height and N application rate. There were no differences in extractable NO3-N between weed heights at 0, 134, and 202 kg/N ha. When grown with 67 kg N/ha, NO3-N content was greater for 10 cm tall weeds than 20 cm weeds. Extractable NO3-N increased with N application rate for both weed heights.
The C:N ratio of weed residue was influenced by the interaction of weed species and N application rate. The C:N ratio of the weed residue ranged between 8.0 to 21.1. Giant foxtail had a greater C:N ratio than the other two weed species when grown with 0 and 67 kg N/ha. When weeds were grown with 134 to 202 kg N/ha, there was no significant difference in C:N ratio among the three weed species. The C:N ratio of weed residue was also influenced by the interaction of weed height and N application rate. When weeds were grown with 0 and 67 kg/N ha, 20 cm tall weeds had a greater C:N ratio than 10 cm tall weeds. There was no difference in C:N ratio of 10 and 20 cm tall weeds species when they were grown with 134 and 202 kg N/ha. C:N ratio of weed residue decreased with increasing N application rate.
Percentage of N mineralized from weed residue increased with N application rate. However, when 0 kg N/ha was applied to giant foxtail, N release was never greater than 13.3 %, which occurred 12 wk after incubation. Nitrogen mineralization from weed residue exceeded 25% for the other treatments by 12 wk of incubation. Percentage of total N mineralized from weed residue was also influenced by the interaction of weed height and N application rate. Younger plant materials will decompose more rapidly than mature plant residues. However, as seen in this study, this difference may be most apparent when weeds are grown with low N application rates.
Rate of N mineralization differed among treatment combinations. The C:N ratio of weed residue was negatively correlated with the rate constant. The first-order rate constant was also correlated with extractable NO3-N content of weeds. However, this correlation (r = 0.55) was not as strong as the correlation (r = -0.79) between the C:N ratio and the rate constant.
Weeds should be controlled prior to reaching 20 cm height, especially under low soil N conditions, to avoid N immobilization from weed residue. Weeds that are 10 cm tall may contribute to the soil N pool; however, this practice may not be recommended. In this study, weeds grew from 10 to 20 cm tall in nine days. If unfavorable weather conditions prevent timely weed control, weeds that reach 20 cm height may negatively affect soil N pools and subsequently impact crop yields.
Educational & Outreach Activities
1. ASA-CSSA-SSSA international annual meeting, 2011, oral presentation
2. North Central Extension-Industry Soil Fertility Conference, 2011, oral presentation
3. North Central Weed Science Society annual meeting, 2011, oral presentation
4. WSSA annual meeting, 2012, poster presentation
1.Diagnostic Day (field day), Saginaw Valley Research and Extension Center, Michigan State University Extension, 2011
2.Conservation Tillage and Technology Conference, The Ohio State University Extension, 2012
1. PhD Dissertation (not completed)
2. Peer-reviewed publication (not completed)
This study increases understanding of the quantity and rate of N mineralization of weed residue under controlled laboratory conditions and helped to develop N recommendations and weed control practices that maintain crop productivity while minimizing N loss to the environment.
Oral presentations were given at the ASA-CSSA-SSSA international annual meeting (2011, San Antonio, TX), the North Central Extension-Industry Soil Fertility Conference (2011, Des Moines, IA), and the North Central Weed Science Society annual meeting (2011, Milwaukee, WI). Results will also be presented as a poster at the 2012 WSSA annual meeting in Hawaii.
In 2011, data were discussed with growers at a field day hosted by Michigan State University Extension at the Saginaw Valley Research and Extension Center. Research results will also be discussed in 2012 at the annual Conservation Tillage and Technology Conference hosted by Ohio State University Extension.
No formal economic analysis was conducted. This study was primarily a laboratory study, making a field-scale economic analysis difficult. This study quantified N mineralization from weed residue under controlled conditions and provided baseline information. Future field studies should be conducted with an appropriate economic analysis.
This study increases understanding of the quantity and rate of N mineralization of weed residue under controlled laboratory conditions and helped to develop N recommendations and weed control practices that maintain crop productivity while minimizing N loss to the environment. Many growers want to better understand the relationship between N applications and weed growth. This study helps answer their questions about rate and quantity of N mineralized from weeds.
Areas needing additional study
Additional field research is needed. This study provided baseline data under controlled conditions. Nitrogen mineralization is dependent on many factors including soil management, temperature, and moisture. Field studies that further investigate N mineralization from weed residue are warranted.