blanket recommendations for fertilizer N are used (Adhikari et al., 1999). Overapplication of N in cereal crops leads to low N recovery effi-When N application is not synchronized with crop ciency and risk of NO 3 pollution of ground water. The chlorophyll demand, N losses from the soil-plant system are large, meter, also known as SPAD meter, is a simple, portable diagnostic leading to low N fertilizer use efficiency. Peng and Casstool for identifying crop N status. We used it to test need-based N management approaches for rice (Oryza sativa L.) and wheat (Triti-821

No abstract

Rice (Oryza sativa L.)–wheat (Triticum aestivum L.) farmers in India burn or remove residues to facilitate seedbed preparation. Incorporation of residues before planting of the next crop generally decreases yields due to N immobilization. Since a window of about 40 d is available between rice harvest and wheat planting, the effect of time of incorporation on rice residue decomposition and N mineralization–immobilization was studied in 1992–1993. The mass loss of residue was 25% for a 10‐d, 35% for a 20‐d, and 51% for a 40‐d decomposition period before wheat planting. Nitrogen release from residue ranged from 6 to 9 kg ha−1 during the wheat season. The immobilization of urea N decreased when residue was allowed to decompose for 10‐d or longer. Based on these studies, a long‐term (1993–2000) experiment was conducted on a sandy loam soil to examine the effect of time of residue incorporation before sowing wheat when compared with burning or removal of residue on yields, N‐use efficiency, and soil fertility. The effect of wheat residue incorporation with green manure (GM, Sesbania cannabina L.) on subsequent rice yields was also determined. Residue incorporation for 10 to 40 d had no effect on wheat yields. Rice yields increased (0.18–0.39 Mg ha−1) when wheat residue was incorporated with GM. Starter N applied at residue incorporation did not influence wheat yields but decreased N recovery efficiency. Physiological efficiency was higher when rice straw was incorporated in wheat and when wheat straw plus GM were incorporated in rice than when rice straw was incorporated for 10 d or when the straw was burned. The long‐term application of rice residue increased C accumulation in soil.

blanket recommendations for fertilizer N are used (Adhikari et al., 1999). Overapplication of N in cereal crops leads to low N recovery effi-When N application is not synchronized with crop ciency and risk of NO 3 pollution of ground water. The chlorophyll demand, N losses from the soil-plant system are large, meter, also known as SPAD meter, is a simple, portable diagnostic leading to low N fertilizer use efficiency. Peng and Casstool for identifying crop N status. We used it to test need-based N management approaches for rice (Oryza sativa L.) and wheat (Triti-man (1998) demonstrated that RE of topdressed urea cum aestivum L.) on a loamy sand in northwestern India. Applying during panicle initiation stage could be as high as 78%. 30 kg N ha Ϫ1 each time the SPAD value fell below the critical value Hence, plant need-based application of N is crucial for of 37.5 resulted in application of 90 kg N ha Ϫ1 , which produced rice achieving high yield and N use efficiency. Soil tests for yields equivalent to those with 120 kg N ha Ϫ1 applied in three splits. N fertilizer recommendations in flooded rice soils have Using a SPAD value of 35 was inadequate for the two rice cultivars not been successful (Stalin et al., 1996; Adhikari et al., because it resulted in application of only 60 kg N ha Ϫ1 and, thus, low 1999). The chlorophyll meter (SPAD-502, Minolta, Ramyields. With high inherent soil fertility resulting in rice yield of Ͼ3 sey, NJ), also known as SPAD (soil plant analysis devel-Mg ha Ϫ1 in zero-N plots, applying N basally or a week after rice opment) meter, can quickly and reliably assess the N transplanting did not further increase yield. Limited experimentation status of a crop based on leaf area. It has been success-821

The sustainability of the rice (Oryza sativa L.)–wheat (Triticum aestivum L.) rotation is important to Asia's food security. Intensive cropping with no return of crop residues and other organic inputs result in the loss of soil organic matter (SOM) and nutrient supply, and is assumed to be nonsustainable. We evaluated seven treatments comprised of various combinations of green manure (GM; Sesbania cannabina L.); wheat straw (WS), farmyard manure (FYM), and urea on yields and yield trends; P and K balance; and soil fertility in a rice–wheat experiment (1988–2000) on a loamy sand in Punjab, India. Rice yields were comparable with GM + urea, WS + GM + urea, and urea alone, but yields were reduced when FYM was supplemented with N. Except during 1 yr, integrated use of FYM and GM produced equal or higher rice yields than other GM based treatments. Wheat straw incorporation reduced average rice yields by 7% compared with WS removal. After 5 yr of continuous application, FYM and WS were at par in increasing rice yields. Organic materials applied to rice had no residual effect on wheat yields except FYM, which increased yield by about 6% compared with urea alone. Rice yield declined by 0.02 to 0.13 Mg ha−1 yr−1 but wheat yields remained unchanged. Soil C increased with the application of WS and FYM. Potassium balance was highly negative. Although the causes of yield decline are unknown, inadequate K applications and changes in the climatic parameters are possible reasons.