Xu Zhao

Nitrogen fate and environmental consequence in paddy soil under rice-wheat rotation in the Taihu lake region, China

Field undisturbed tension-free monolith lysimeters and 15N-labeled urea were used to investigate the fate of fertilizer nitrogen in paddy soil in the Taihu Lake region under a summer rice-winter wheat rotation system. We determined nitrogen recovered by rice and wheat, N remained in soil, and the losses of reactive N (i.e., NH3, N2O, NO3−, organic N and NH4+) to the environment. Quantitative allocation of nitrogen fate varied for the rice and wheat growing seasons. At the conventional application rate of 550xa0kg N ha−1 y−1 (250xa0kg N ha−1 for wheat and 300xa0kg N ha−1 for rice), nitrogen recovery of wheat and rice were 49% and 41%, respectively. The retention of fertilizer N in soil at harvest accounted for 29% in the wheat season and for 22% in the rice season. N losses through NH3 volatilization from flooded rice paddy was 12%, far greater than that in the wheat season (less than 1%), while N leaching and runoff comprised only 0.3% in the rice season and 5% in the wheat season. Direct N2O emission was 0.12% for the rice season and 0.14% for the wheat season. The results also showed that some dissolved organic N (DON) were leached in both crop seasons. For the wheat season, DON contributed 40–72% to the N- leaching, in the rice season leached DON was 64–77% of the total N leaching. With increasing fertilizer application rate, NH3 volatilization in the rice season increased proportionally more than the fertilizer increase, N leaching in the wheat season was proportional to the increase of fertilizer rate, while N2O emission increased less in proportion than fertilizer increase both in the rice season and wheat season.

Methane emissions from a rice agroecosystem in South China: Effects of water regime, straw incorporation and nitrogen fertilizer

To quantitatively assess the effects of agricultural practices on methane (CH4) emissions from rice fields, a two-year (2005/2006) field experiment with 23 factorial designs was conducted to assess the effects of three driving factors on CH4 emissions in South China: continuously flooded (W0) and mid-season and final drainages (W2), straw (S1) and nitrogen fertilizer (N1) applications and their controls (S0, N0). Results showed that averaged across all the treatments about 75xa0% of the seasonal total CH4 occurred between the rice transplanting and booting stage, while constituted only 33xa0% of the seasonal total rice biomass during the same period. Averaged across the treatments in 2006, CH4 emissions were substantially decreased by mid-season drainage up to 60xa0% (15.6 vs. 39.0xa0gxa0m−2). The decreased CH4 emissions represented almost all of the decrease in the total global warming potentials. Without straw incorporation CH4 emissions substantially decreased up to 59xa0% (15.9 vs. 38.7xa0gxa0m−2). The stimulating effects of straw were significantly greater for W0 than W2 treatment, being also greater in the 2005 than in the 2006 season. A significant inter-annual difference in CH4 emissions was found when averaged across straw incorporation and N fertilizer applications for the W2 treatment (42.8 and 15.4xa0gxa0m−2 in 2005 and 2006, respectively). Moreover, N fertilization has no significant effect on CH4 emissions in this study. Our results demonstrate that although straw effects varied greatly with specific management, both straw managements and water regimes are equally important driving factors and thus being the most promising measures attenuating CH4 emissions while achieving sustainable rice production.

Improving grain yield and reducing N loss using polymer-coated urea in southeast China

The efficiency of classical mineral NPK fertilizers is usually low because a major part of these fertilizers does not reach plant roots and ends up polluting groundwaters with nitrates and phosphates. Recently, a novel polymer-coated urea made from recycled plastics was proposed to enhance N availability in cereal production. To evaluate the efficiency of this polymer for rice production, we set up field plots, microplots, and pot experiments with 15N tracing. We compared rice yield, N uptake, and N loss between conventional three split applications of urea and a single basal application of four derivatives from the polymer-coated urea. The four derivatives included a blend with 70xa0% of N from 6xa0% (w/w) coated urea and 30xa0% from urea and three coated urea fertilizers with 6, 8, and 12xa0% coating at an identical N application rate during two rice-growing seasons. Results show that 6xa0% coated urea improved 15N recovery, reduced 15N loss, and increased grain yield slightly due to an initial 15N burst occurring at high field temperatures after basal fertilization; 8 or 12xa0% coated urea better met plant N demand from transplanting to heading, greatly enhanced 15N recovery, and decreased 15N loss and NH3 volatilization. Nevertheless, unlike a significant increase of yield for 12xa0% coated urea, 8xa0% coated urea did not increase yield due to 15N release and excessive 15N uptake by plants at ripening. Overall, our findings show that a single basal polymer-coated urea application improves N use efficiency and reduces N loss in rice agroecosystem.

A five-year P fertilization pot trial for wheat only in a rice-wheat rotation of Chinese paddy soil: interaction of P availability and microorganism

Background and aimsThe need for efficient use of phosphorus (P) in agriculture has been highlighted recently by concerns about the finite amount of P fertilizer resources. However, in the Taihu Lake Region (TLR) of China, farmers’ injudicious and excessive use of P fertilizer has led to a dramatic spike in P accumulation.MethodsA five-year (ten consecutive crop seasons) pot experiment was conducted using four paddy soils with three P concentrations (2 P-rich, 1 P-moderate, and 1 P-deficient soils) from the TLR under four P fertilization regimes: P fertilization only for the wheat season (PW), P fertilization only for rice season (PR), P fertilization for both rice and wheat seasons (PRu2009+u2009W), and no P fertilization during either season (Pzero; control).ResultsOver 5xa0years, compared to the PRu2009+u2009W treatment, the PW treatment did not decrease crop yield (Pu2009<u20090.05) because it could supply enough available P sources (124–210xa0mgxa0kg-1 labile P and moderately labile P) for crop growth and similar microorganism community composition. Also, compared to the Pzero treatment, applied P fertilization significantly increased the concentration of labile P and moderately labile P. Additionally, applied P fertilization decreased acid phosphatase enzyme activity and increased the total relative abundance of microorganisms significantly in P-rich soil, although they decreased in P-deficient soil. Arbuscular mycorrhizal fungi (AMF) showed significant positive correlations with soil labile P (Pu2009<u20090.05), which indicated that AMF played important roles in the transformation of P in the soil P pool.ConclusionsP fertilizer applied only for the wheat season may be a viable option for saving P fertilizer and sustaining crop yields in the current rice-wheat rotated system of China, and effective utilization of AMF which are related to P availability in the soil will be important in the future reasearch.

The regime and P availability of omitting P fertilizer application for rice in rice/wheat rotation in the Taihu Lake Region of southern China

PurposeIn the Taihu Lake Region (TLR) of China, farmers’ injudicious and excessive use of phosphorus (P) fertilizer has led to a dramatic spike in P accumulation. In view of that, the water flooding practice can increase soil P release and enhance P availability in rice season, compared with the strong P fixation in wheat season; it seems possible to save P fertilizer in rice season with the aim of reducing P loads without any crop yield declines.Materials and methodsTo validate this possibility, a 4-year pot experiment encompassing eight rice/wheat seasons and using four paddy soils with varying Olsen-P contents (6.16 to 40.95xa0mgxa0kg−1) was conducted to compare rice/wheat yield, inorganic and organic P accumulation under four different P regimes, P fertilization for both rice and wheat (PRu2009+u2009W; conventional practice), P fertilization only for wheat (PW), P fertilization only for rice (PR), and no P fertilization for both seasons (Pzero).Results and discussionCompared with conventional PRu2009+u2009W treatment, PR treatment significantly decreased wheat yields, especially in medium- and low-P soils, with an Olsen-P concentrate decline of 34.4–62.8xa0%. In contrast, PW treatment showed no significant difference in the rice/wheat yields over 4xa0years irrespective of high-, medium-, and low-P-concentrated soils, despite the soil Olsen-P concentration declining by 34.9–64.4xa0%. This highlights the feasibility of omitting P fertilizer application to flooded rice for at least 4xa0years in rice/wheat cropping paddy fields while maintaining crop yields and reducing environmental risk. In four paddy soils, available inorganic P was the dominant effective P source and increased with the concentration of Olsen-P. Without P fertilization over time, the concentration of soil inorganic P fractions declined and organic P remained relatively constant.ConclusionsAccording to the P supply capacity of different soils under the regime of omitting P fertilization for rice, how to utilize the bioavailability of P in different P supply capacity soils when P fertilization is omitted for rice crops will be required in future work.

Use of Nitrogen Isotope To Determine Fertilizer- and Soil-Derived Ammonia Volatilization in a Rice/Wheat Rotation System

The nitrogen (N) isotope method reveals that application of fertilizer N can increase crop uptake or denitrification and leaching losses of native soil N via the added N interaction. However, there is currently little evidence of the impact of added N on soil N losses through NH3 volatilization using (15)N methodologies. In the present study, a three-year rice/wheat rotated experiment with 30% (15)N-labeled urea applied in the first rice season and unlabeled urea added in the following five crop seasons was performed to investigate volatilization of NH3 from fertilizer and soil N. We found 9.28% of NH3 loss from (15)N urea and 2.88-7.70% declines in (15)N-NH3 abundance occurred during the first rice season, whereas 0.11% of NH3 loss from (15)N urea and 0.02-0.21% enrichments in (15)N-NH3 abundance happened in the subsequent seasons. The contributions of fertilizer- and soil-derived N to NH3 volatilization from a rice/wheat rotation were 75.8-88.4 and 11.6-24.2%, respectively. These distinct variations in (15)N-NH3 and substantial soil-derived NH3 suggest that added N clearly interacts with the soil source contributing to NH3 volatilization.

Nitrogen removal from the surface runoff of a field scale greenhouse vegetable production system

Nutrient losses from greenhouse vegetable production systems may impair water quality in the Taihu Lake Region of China. We studied the characteristics of nitrogen (N) lost via runoff from greenhouse vegetable systems and strategies for minimizing N entering water bodies. A two-year experiment at a field scale was conducted to monitor N surface runoff. An eco-ditch (148u2009m2) and a low N input paddy field (135u2009kgu2009Nu2009ha−1, 550u2009m2) were designed to remove N from the surface runoff of a 25u2009×u200950u2009m greenhouse vegetable field. The greenhouse was not covered from late June to mid-October each year, and runoff occurred multiple times during this period. Annual total N loss in runoff from the greenhouse vegetable site was 25.3 and 33.5u2009kgu2009ha−1 in 2010 and 2011, respectively. Nitrate-N was the major form of N lost in the runoff. The average runoff volume was 289u2009mm (varied from 221 to 357u2009mm), which contained 15.7 (varied from 3.3 to 39.2u2009mgu2009L−1) mg L−1 total N. The eco-ditch system and the wetland paddy field (WPF) effectively reduced total N discharge; the removal rates reached 49.9% and 58.7% and the average removal capacities were 12.4u2009gu2009Nu2009m−2 and 4.1u2009gu2009Nu2009m−2 in 2010 and 2011, respectively. The combined system of the ecological ditch–WPF removed almost 79% total N in the runoff. Ecological ditch or paddy wetland can be a water management option available to growers in this region to economically reduce pollutants in agricultural runoff.