Marco Roelcke

Nitrogen Fertilization, Soil Nitrate Accumulation, and Policy Recommendations in Several Agricultural Regions of China

Abstract Excessive nitrogen (N) fertilization and decreasing N recovery rates by crops have caused dramatic increases in non-point source pollution from agriculture in China. The rate of N fertilization across the country varies widely among regions and crops, depending on the stage of economic development. For example, N application rates in the eastern regions and on cash crops are far higher than in western regions of the country and on cereal crops. Moreover, N application rates in wealthier regions are higher than recommended by the Chinese Academy of Sciences. To successfully achieve environmental protection as well as high crop yields, China must formulate relevant agricultural policies to encourage farmers in economically developed areas to reduce their N fertilization rate while also issuing conventional fertilization recommendations for small-scale farming systems and the expanding cultivation of cash crops.

The N-cycle as determined by intensive agriculture - examples from central Europe and China.

Using a scientific assessment concept of sustainability in crop-production based on the entropy production minimization principle of thermodynamics, formation and non-use of soluble and volatile (by-)products of the nutrient cycles within the system are interpreted as indicators or measures of the low efficiency/sustainability of recent forms of intensive agriculture. The simultaneous high energy input in modern crop production systems further shows the difference between these and quasi-stationary natural systems with maximum bioproduction having minimum energy dissipation and entropy production. Using balance sheets and dynamic approaches, the practical implications regarding the nitrogen cycle in central Europe (FR Germany) and China are exemplified and discussed. The average N balance of arable systems in Germany shows surplus N amounts of 110–130 kg N ha-1 yr-1. A high N immobilization in accordance with deepened top soil layers has governed N balances in Germany since about 1960. In China Nbalance surpluses in intensive agricultural (double-cropping) systems on the southern edge of the Loess Plateau now reach 125–230 kg N ha-1 yr-1. In field experiments, mineral N contents in the profiles (0–1.2 m depth) were 72–342 and 78–108 kg ha-1 at harvest of summer maize and winter wheat, respectively. In the Taihu region in eastern China, surpluses in the N balance (rice-wheat double cropping) amount to 217–335 kg N ha-1 yr-1. Nmin contents in the 0–0.9 m profiles of between 50 and 100 kg N ha-1 were frequently found after winter wheat harvest. In two separate investigations of ground and well water samples in China, nitrate contents exceeded the critical WHO value for drinking water in 38–50% of the locations investigated.

Utilization and management of organic wastes in Chinese agriculture: Past, present and perspectives

Recycling and composting of organic materials such as animal waste, crop residues and green manures has a long tradition in China. In the past, the application of organic manures guaranteed a high return of organic materials and plant mineral nutrients and thus maintained soil fertility and crop yield. As a result of rapid economic development coupled with the increasing urbanization and labour costs, the recycling rate of organic materials in Chinese agriculture has dramatically declined during the last two decades, in particular in the more developed eastern and southeastern provinces of China. Improper handling and storage of the organic wastes is causing severe air and water pollution. Because farmers are using increasing amounts of mineral fertilizer, only 47% of the cropland is still receiving organic manure, which accounted for 18% of N, 28% of P and 75% of K in the total nutrient input in 2000. Nowadays, the average proportion of nutrients (N+P+K) supplemented by organic manure in Chinese cropland is only 35% of the total amount of nutrients from both inorganic and organic sources.In China, one of the major causes is the increasing de-coupling of animal and plant production. This is occurring at a time when “re-coupling” is partly being considered in Western countries as a means to improve soil fertility and reduce pollution from animal husbandry. Re-coupling of modern animal and plant production is urgently needed in China. A comprehensive plan to develop intensive animal husbandry while taking into account the environmental impact of liquid and gaseous emissions and the nutrient requirements of the crops as well as the organic carbon requirements of the soil are absolutely necessary. As a consequence of a stronger consideration of ecological aspects in agriculture, a range of environmental standards has been issued and various legal initiatives are being taken in China. Their enforcement should be strictly monitored.

Calibration of a simple method for determining ammonia volatilization in the field – comparative measurements in Henan Province, China

The determination of ammonia volatilization with sufficient spatial and temporal resolution requires a simple and versatile in situ measurement technique, particularly in developing countries. Therefore, a simple chamber method for determining ammonia (NH3) volatilization in the field (Dräger-Tube Method; DTM) was calibrated by comparison with simultaneous measurements with a micrometeorological Integrated Horizontal Flux (IHF) method. Five field experiments were conducted following urea fertilization on summer maize and winter wheat plots (1998–1999) at Fengqiu Experimental Station, Central China. The simplicity of the chamber method allowed for measurements to be carried out by trained farmers. The measurements with both methods yielded very similar patterns of NH3 fluxes and similar differences between fertilization treatments. Cumulative NH3 losses determined by the IHF method ranged from 14.6 to 47.9% and from 0.6 to 17.9% of urea-N applied for surface broadcast and incorporated fertilization, respectively. As expected, cumulated NH3 losses were underestimated by the DTM as compared to the IHF by about one order of magnitude. A calibration equation was calculated by multiple linear regression which included NH3 flux data as well as temperature and wind speed values. The calibration model yielded a modelling efficiency c2 of 0.86 resulting in an average estimation error of cumulative NH3 losses of 17%. The equation was validated by comparison of IHF measurements and DTM fluxes not considered in the derivation of the calibration formula. The calibration approach can be used under similar meteorological and field conditions irrespective of the soil characteristics or type of N fertilizer applied.

The effect of fertilizer placement on nitrogen uptake and yield of wheat and maize in Chinese loess soils

Field trials were carried out to study the fate of15N-labelled urea applied to summer maize and winter wheat in loess soils in Shaanxi Province, north-west China. In the maize experiment, nitrogen was applied at rates of 0 or 210 kg N ha−1, either as a surface application, mixed uniformly with the top 0.15 m of soil, or placed in holes 0.1 m deep adjacent to each plant and then covered with soil. In the wheat experiment, nitrogen was applied at rates of 0, 75 or 150 kg N ha−1, either to the surface, or incorporated by mixing with the top 0.15 m, or placed in a band at 0.15 m depth. Measurements were made of crop N uptake, residual fertilizer N and soil mineral N. The total above-ground dry matter yield of maize varied between 7.6 and 11.9 t ha−1. The crop recovery of fertilizer N following point placement was 25% of that applied, which was higher than that from the surface application (18%) or incorporation by mixing (18%). The total grain yield of wheat varied between 4.3 and 4.7 t ha−1. In the surface applications, the recovery of fertilizer-derived nitrogen (25%) was considerably lower than that from the mixing treatments and banded placements (33 and 36%). The fertilizer N application rate had a significant effect on grain and total dry matter yield, as well as on total N uptake and grain N contents. The main mechanism for loss of N appeared to be by ammonia volatilization, rather than leaching. High mineral N concentrations remained in the soil at harvest, following both crops, demonstrating a potential for significant reductions in N application rates without associated loss in yield.

In situ comparisons of ammonia volatilization from N fertilizers in Chinese loess soils

Ammonia volatilization loss from mineral N fertilizers was determined on a calcareous Chinese loess soil with a pH (CaCl2) of 7.7. An original in situ method that required no electricity or laboratory analyses was used. By means of a bellows pump, ambient air was drawn through four conical cups placed onto the soil (total area 400 cm2) and subsequently through an NH3-specific detector tube with direct colorimetric indication of the ammonia concentration (measuring range, 0.05–700 vol.-ppm NH3). Duration of measurement was about 3 min. Following N fertilization to winter wheat in 1990 and to summer maize in 1991, the application methods surface broadcast, uniform incorporation into the 0–15-cm layer, and for maize, a point placement at 10 cm depth were investigated. Ammonium bicarbonate and urea were applied at rates of 100 and 200 kg N ha−1. In the autumn of 1990, ammonia losses following NH4HCO3 application were more than twice as large as with urea, fertilizer incorporation reduced NH3 losses 15-fold, and doubling the nitrogen application rate resulted in a 1.7-fold increase in the percentage of nitrogen loss. Cumulative ammonia fluxes were about 2 times higher in the summer of 1991. Comparing application methods in summer, losses were significantly (3 times) lower only with point placement. The above differences were all significant at the P<0.05 level. Due to the very low air exchange rate (0.9 volumes min−1), actual volatilization rates were underestimated by this method. Though not yielding absolute amounts, the Dräger-Tube method proved very suitable for comparing relative differences in ammonia fluxes. The measurements clearly reflected the characteristic flux patterns for the different treatments and the effects of environmental factors on their time course.

Nitrogen mineralization in paddy soils of the Chinese Taihu Region under aerobic conditions

In order to make more efficient use of mineral nitrogen fertilizers, the mineralization of organic N has to be fully understood and taken into account when meeting the nitrogen demand of crops. Aerobic long-term incubation experiments (147 days) based on the method by Stanford and Smith (1972, Soil Sci Soc Am Proc 36: 465), modified by Nordmeyer and Richter (1985, Plant Soil 39: 433), were carried out with paddy soils (Typic Epiaquepts) from two different locations in the Taihu Region in Eastern China, each in three depth increments (0–30, 30–60, 60–90 cm). The two soils had pH (H2O) values between 6 and 7, silty clay loam texture, Corg contents (0–30 cm) of 8.9 and 16.6 g kg−1 and Ntot contents of 1030 and 1650 mg kg−1, respectively. Mineralization parameters were estimated from the cumulative amounts of leached nitrate-N via non-linear regression, using a two-pool approach based on first-order kinetics reactions. The results were compared to those from incubation experiments carried out with topsoils from the southern edge of the Chinese Loess Plateau (Roelcke et al., 1996, Trans. 9th Nitrogen Workshop, Braunschweig, p, 313). The optimized parameters were used to simulate the N regime in paddy soils with a deterministic model (Han et al., 2001, Proc. Int. Conf. China, p. 411). A winter wheat growing period (November 1995 to June 1996) in Wuxi County was simulated. A total of 220 kg ha−1 mineral N fertilizer were applied as NPK and urea, split into four doses. Simulated results were compared to the measured mineral nitrogen (Nmin) contents over the growing season. Cumulative aerobic N mineralization (0–1.0 m profile) yielded 84 kg N ha−1. The modelled total cumulative N losses amounted to 76 kg N ha−1 or 35% of the N applied.

Sources of nitrous and nitric oxides in paddy soils: Nitrification and denitrification

Rice-paddies are regarded as one of the main agricultural sources of N 2O and NO emissions. To date, however, specific N2O and NO production pathways are poorly understood in paddy soils. (15)N-tracing experiments were carried out to investigate the processes responsible for N2O and NO production in two paddy soils with substantially different soil properties. Laboratory incubation experiments were carried out under aerobic conditions at moisture contents corresponding to 60% of water holding capacity. The relative importance of nitrification and denitrification to the flux of N2O was quantified by periodically measuring and comparing the enrichments of the N2O, NH(+)4-N and NO(-)3-N pools. The results showed that both N2O and NO emission rates in an alkaline paddy soil with clayey texture were substantially higher than those in a neutral paddy soil with silty loamy texture. In accordance with most published results, the ammonium N pool was the main source of N2O emission across the soil profiles of the two paddy soils, being responsible for 59.7% to 97.7% of total N2O emissions. The NO(-)3-N pool of N2O emission was relatively less important under the given aerobic conditions. The rates of N2O emission from nitrification (N2On) among different soil layers were significantly different, which could be attributed to both the differences in gross N nitrification rates and to the ratios of nitrified N emitted as N2O among soil layers. Furthermore, NO fluxes were positively correlated with the changes in gross nitrification rates and the ratios of NO/N2O in the two paddy soils were always greater than one (from 1.26 to 6.47). We therefore deduce that, similar to N2O, nitrification was also the dominant source of NO in the tested paddy soils at water contents below 60% water holding capacity.

A new urease-inhibiting formulation decreases ammonia volatilization and improves maize nitrogen utilization in North China Plain

Overuse of urea, low nitrogen (N) utilization, and large N losses are common in maize production in North China Plain (NCP). To solve these problems, we conducted two field experiments at Shangzhuang and Quzhou in NCP to test the ability of a newly developed urease inhibitor product Limus® to decrease NH3 volatilization from urea applied to maize. Grain yield, apparent N recovery efficiency (REN) and N balance when using urea applied with or without Limus were also measured over two maize growing seasons. Cumulative NH3 loss in the two weeks following urea application without Limus ranged from 9–108 kg N ha−1, while Limus addition significantly decreased NH3 loss by a mean of 84%. Urea with Limus did not significantly increase maize yields (P < 0.05) compared with urea alone. However, a significant 11–17% improvement in REN with Limus was observed at QZ. The use of urea-N plus Limus would permit a reduction in N applications of 55–60% compared to farmers’ practice and/or further 20% N saving compared with optimized urea-N rate (150 kg N ha−1, based on N requirement by target yield of 7.5 t ha−1), and would achieve the same maize yields but with significantly decreased NH3 loss and increased N utilization.

Greenhouse gas emissions from the enteric fermentation and manure storage of dairy and beef cattle in China during 1961–2010

Due to the expanding dairy and beef population in China and their contribution to global CH4 and N2O budgets, a framework considering changes in feed, manure management and herd structure was established to indicate the trends of CH4 and N2O emissions from the enteric formation and manure storage in China׳s beef and dairy production and the underlying driving forces during the period 1961-2010. From 1961 to 2010, annual CH4 and N2O emissions from beef cattle in China increased from 2.18Mt to 5.86Mt and from 7.93kt-29.56kt, respectively, while those from dairy cattle increased from 0.023 to 1.09Mt and 0.12 to 7.90kt, respectively. These increases were attributed to the combined changes in cattle population and management practices in feeds and manure storage. Improvement in cattle genetics during the period increased the bodyweight, required dry matter intake and gross energy and thus resulted in increased enteric CH4 EFs for each category of beef and dairy cattle as well as the overall enteric EFs (i.e., Tier 1 in IPCC). However, for beef cattle, such an impact on the overall enteric EFs was largely offset by the herd structure transition from draft animal-oriented to meat animal-oriented during 1961-2010. Although the CO2-eq of CH4 and N2O from manure storage was less than the enteric emissions during 1961-2010 in China, it tended to increase both in beef and dairy cattle, which was mainly driven by the changes in manure management practices.