M. I. Samson

Long-term Comparison of the Agronomic Efficiency and Residual Benefits of Organic and Inorganic Nitrogen Sources for Tropical Lowland Rice

Nitrogen efficiency from Azolla microphylla or Sesbania rostrata green manure, rice straw, and inorganic fertilizer-N was compared in two long-term experiments with irrigated lowland rice ( Oryza sativa L.). Treatments included a control and each nitrogen source alone or in combinations that provided 50% of the total applied nitrogen from an organic and inorganic nitrogen source. All nitrogen sources were applied at equivalent nitrogen rates to 19–22 consecutive rice crops. Residual effects were assessed in two subsequent cropping seasons at one site. Lower grain yield, agronomic efficiency (Δgrain per kg total applied nitrogen), and apparent nitrogen uptake were obtained from green manure and rice straw nitrogen as sole or dual nitrogen sources rather than from a standard split application of prilled urea. Compared to prilled urea, residual effects from green manure or rice straw included a significant increase in soil organic carbon and total nitrogen, and greater extractable soil nitrogen in the vegetative growth period. After panicle initiation there was no residual effect on the rate of crop nitrogen accumulation, and final grain yields were similar regardless of previous nitrogen source. Recycling of rice straw appeared to have greater potential for reducing fertilizer-N requirements than use of green manure because rice straw is often a wasted resource in irrigated rice systems of the humid tropics, the efficiency of rice straw nitrogen in combination with prilled urea is comparable to green manure nitrogen, and the increase in soil nitrogen from rice straw was 50–150% greater than from green manure.

Evolution and soil entrapment of nitrogen gases formed by denitrification in flooded soil

Abstract The direct field measurement of N loss by denitrification in flooded soils has been a long-standing challenge. A field experiment was conducted in puddled, flooded soil without plants to ascertain whether the directly measured evolution of (N2 + N2O)-15N from 15N-labeled nitrate applied to floodwater was an accurate measure of denitrification loss. The evolution of (N2 + N2O)-15N was determined from the 15N content of air samples collected in a chamber placed over the floodwater. The evolved (N2 + N2O)-15N increased for 2 to 3 days following addition of either 3.5, 6.9, or 10.4 kg nitrate-N-ha-1 and then decreased on succeeding days. Evolved (N2 + N2O)-15N correlated (r = 0.86) with floodwater (NO3 - + NO2 -)-N measured 48 h earlier. The total (N2 + N2O)-15N evolved over the 10 days following nitrate addition ranged from 20 to 25% of the added N. The added 15N not recovered in the soil and floodwater after 10 days ranged from 48 to 64% of the added N. This unrecovered 15N presumably represented d...

Quantification of denitrification in flooded soils as affected by rice establishment method

Abstract The recovery of 15 N-labeled N 2 and N 2 O evolved from added 15 N-labeled fertilizers can reportedly underestimate N loss by denitrification in puddled, flooded soils. Field research was conducted for 2 yr in the Philippines to determine whether the evolution of N gases formed by denitrification is influenced by a rice ( Oryza sativa L.) establishment method. Quantification of denitrification was assessed by comparing the cumulative recovery of (N 2 + N 2 O)- 15 N evolved from added 15 N-labeled nitrate (5 kg N ha −1 ) with the total N loss by denitrification, which was estimated from the added NO 3 − - 15 N not recovered in the 15 N balance at the conclusion of the 20 day gas collection period. In both years, rice was established by either transplanting (TPR) at 20 × 20 cm spacing between hills with three seedlings per hill (75 plants m −2 ) or wet broadcast seeding of germinated seed (BSR) with a plant population of 370 m −2 in 1989 and 250 m −2 in 1990. In 1990, wet seeding of germinated seed in 20-cm-wide rows (RSR) with 250 plants m −2 was also included. Nitrate addition and gas collection commenced at 11 days after transplanting (DT) 20 or 21-days-old seedlings and 21 days after wet sowing (DS). In 1989, the recovery of added N as evolved (N 2 + N 2 O)- 15 N (54% for BSR and 47% for TPR) was comparable to total denitrification loss for BSR (53%) but not for TPR (61%). In 1990, the recovery of evolved (N 2 + N 2 O)- 15 N (32% for TPR, 61% for BSR and 40% for RSR) underestimated denitrification loss (72% for TPR and 73% for BSR and RSR), but to a lesser extent for BSR than for TPR and RSR. The greater recovery of added N as evolved (N 2 + N 2 O)- 15 N when chambers were placed over transplated rice rather than between plants (47 and 40%, respectively) suggested that rice was a conduit for transport of gas from soil. Underestimation of denitrification was attributed to entrapment of N-gases in soil. Results suggest that young BSR, because of high plant density and rapid extension of roots into the soil volume, was a more effective conduit for gas transport than TPR or RSR.

Contribution of fallow periods between rice crops to seasonal GHG emissions: effect of water and tillage management

ABSTRACT Irrigated rice cultivation is a major source of greenhouse gas (GHG) emissions from agriculture. Methane (CH4) and nitrous oxide (N2O) are emitted not only throughout the growing season but also in the fallow period between crops. A study was conducted for two transition periods between rice crops (dry to wet season transition and wet to dry season transition) in the Philippines to investigate the effect of water and tillage management on CH4 and N2O emissions as well as on soil nitrate and ammonium. Management treatments between rice crops included (1) continuous flooding (F), (2) soil drying (D), (3) soil drying with aerobic tillage (D + T), and (4) soil drying and wetting (D + W). The static closed chamber method was used to measure CH4 and N2O fluxes. Soil nitrate accumulated and N2O was emitted in treatments with soil drying. Nitrate disappeared while ammonium gradually increased after the soil was flooded during land preparation, indicating net nitrogen mineralization. N2O emissions were highest in both transition periods in D + W (437 and 645 µg N2O m−2 h−1). Methane emissions were significant in only the F treatment. The highest global warming potential (GWP) in the transition between rice crops occurred in F, with CH4 contributing almost 100% to the GWP. The GWP from other treatments was lower than F, with about 60–99% of the GWP attributed to N2O emissions in treatments with soil drying. The GWP in the transition between rice crops represented up to 26% of the total GWP from harvest to harvest. This study demonstrates that the transition period can be an important source of GHG emissions with relative importance of CH4 and N2O depending on the soil water regime. Therefore, the transition period should not be disregarded when estimating GHG emissions for rice cropping systems.