Multiseason Recoveries of Organic and Inorganic Nitrogen‐15 in Tropical Cropping Systems

Abstract

In tropical agroecosystems, limited N availability remains a major impediment to increasing yield. A 15N-recovery experiment was conducted in 13 diverse tropical agroecosystems. The objectives were to determine the total recovery of one single 15N application of inorganic or organic N during three to six growing seasons and to establish whether the losses of N are governed by universal principles. Between 7 and 58% (average of 21%) of crop N uptake duringthe first growing season was derived from fertilizer. On average, 79% of crop N was derived from the soil. When 15N-labeled residues were applied, in the first growing season 4% of crop N was derived from the residues. Average recoveries of 15N-labeled fertilizer and residue in crops after the first growing season were 33 and 7%, respectively. Corresponding recoveries in the soil were 38 and 71 %. An additional 6% of the fertilizer and 9.1 % of the residue was recovered by crops during subsequent growing seasons. There were no significant differences in total 15N recovery (average 54%) between N from fertilizer and N from residue. After five growing seasons, more residue N (40%) than fertilizer N (18%) was recovered in the soil, better sustaining the soil organic matter N content. Long-term total recoveries of 15N-labeled fertilizer or residue in the crop and soil were similar. Soil N remained the primary source of N for crops. As higher rainfall and temperature tend to cause higher N losses, management practices to improve N use efficiency and reduce losses in wet tropical regions will remain a challenge.

Contribution of nitrogen from sugarcane harvest residues and urea for crop nutrition

“…After the plant cane, the N fertilizer was turned over into other forms of organic N in soil (SOM, roots, microorganisms) but remained as a pool of available N for the crop, as pointed by Hemwong et al (2009) andDourado Neto et al (2010). Leaching losses of fertilizer N in plant cane grown in Brazil were assumed to be insignificant as observed by Ghiberto et al (2009) using 15 N-enriched fertilizer in a similar study but carried out in a much sandier soil.…”

“…partitioned into three pools: mineral N (soil and fertilizer), humic acids and immobilized N in soil microbial biomass, hence there is a continuous transformation of N among these soil-plant system pools (Dourado Neto et al, 2010). Nitrogen from above ground residues had the same behavior as N-urea, mainly during plant cane growth (higher N recovery), with lower recoveries in subsequent ratoons (Table 2).…”

“…São Paulo State is the largest producer in Brazil, where more than 70 % of the overall sugarcane production is managed as green cane under trash blanket (GCTB) or without trash (or straw) burning prior to harvest (Lucon and Goldemberg, 2010).The GCTB provides annually 5 to 20 Mg ha -1 of remains which contribute to increase soil carbon (C), organic matter (Dourado Neto et al, 2010) and to enhance soil physico-chemical and biological properties (Spain and Hodgen, 1994). Further, in field renewal areas, the shoots and roots from the previous crop are also part of the trash and conserve nutrients in the soil-plant system (Franco et al, 2011;Fortes et al, 2012;Hemwong et al, 2009).…”

“…Remarkably, most of these citations considered one or two cropping cycles and multitemporal results such as that published by Dourado Neto et al (2010) are uncommon. Those authors studied the recovery of N from fertilizers and residues by tropical crops during three to six consecutive cycles in various locations and concluded that crop residues are vital for yield maintenance on those highly weathered environments.…”

Impact of sugarcane trash on fertilizer requirements for São Paulo, Brazil

“…The main impact will occur in the long term. Accordingly to Dourado-Neto et al (2010), although residue-N was not an important source of N to the crop in the first growing season, its contribution to soil organic matter maintenance was about twice that of fertilizer N. Therefore, the long-term effect of crop residues on providing mineral N to the crop may be greater than that of N-fertilizer (Dourado-Neto et al, 2010), indicating the importance of maintaining sugarcane trash over the soil under green harvesting systems.…”