Soil NH_4^+ Fixation and Fertilizer N Recovery as Affected by Soil Moisture and Fertilizer Application Methods

Ammonium fixation and the effects of soil moisture and application methods on fertilizer N recovery were investigated in two soils of Shaanxi Province,China,a Luvisol and an Entisol,through two experiments performed in the laboratory and in a glass shelter,respectively,by using ammonium bicarbonate (NH4HCO3). The laboratory closed incubation box experiment was conducted using the Luvisol to study NH4+ fixation rate at soil moisture levels of 10.1%,22.7% and 35.3% water filled pore space (WFPS). The fixed NH4+-N increased dramatically to 51% and 66%,67% and 74%,and 82% and 85% 1,2 and 36 h after fertilizer incorporation at moisture levels of 10.1% and 22.7% WFPS and 35.3%WFPS,respectively. The rapid NH4+ fixation rates at all moisture levels could help prevent NH4+ losses from ammonia volatilization. In the glass shelter pot experiment,N fertilizer was applied by either banding (in a concentrated strip)or incorporating (thoroughly mixing) with the Entisol and the Luvisol. An average of 74.2% of the added N fertilizer was recovered 26 days after application to the Luvisol,while only 61.4% could be recovered from the Entisol,due to higher NH4+ fixation capacity of the Luvisol. The amount of fixed NH4+ decreased with increasing WFPS. The amount of fixed NH4+ in the incorporated fertilizer treatment was,on average,10% higher than that in the banded treatment.Higher NH4+ fixation rates could prevent N loss and thus increase N recovery. The results from the Luvisol showed lower nitrogen recovery as soil moisture level increased,which could be explained by the fact that most of the fixed NH4+ was still not released when the soil moisture level was low. When the fertilizer was incorporated into the soil,the recovery of N increased,compared with the banded treatment,by an average of 26.2% in the Luvisol and 11.2% in the Entisol,which implied that when farmers applied fertilizer,it would be best to mix it well with the soil.

Nitrogen Uptake by Corn and N Recovery in Grainin Dry Farmland

There have been considerable concerns with the reduced beneficial yield response to fertilizer and the less efficient use of fertilizers with the fast growth in fertilizer consumption in China, as well as problems with low and unstable fertilizer use effectiveness in dry farmland. The paper discusses the effect of precipitation , corn stover incorporation and fertilizer management on N uptake by corn and fertilizer N use efficiency in dry farmland. The results showed that in the areas with 520 mm rainfall under the cropping system of one crop of spring corn per year, N uptake and N recovery by corn, and yields were higher at rates of 105 kg fertilizer N, 1 500 kg cattle manure and 6 000 kg corn stover incorporated per hectare. N uptake was significantly affected by precipitation during growing periods and soil moisture at sowing, fertilizer N apparent recovery varied from 58% in a rainy year to 7% in the year with a very dry soil, and the average of 7 years(1993 -1999)was about 30%. The 15N tracer experiment showed that around 40% of N uptake was derived form fertilizer N and 60% from soil N. The results from 3-year 15N tracer study(1997 - 1999)indicated that the 1st year's N recovery in grain with and without stover incorporated was about 24% and 17%, respectively, the accumulative 3-year total about 36% and 25%, the percentage of N recovery with stover incorporated increased about 11. The accumulative 3-year N loss with and without stover incorporated was about 26% and 45%, respectively, the percentage of N loss with stover decreased about 19. The studies supply information on land applications of fertilizer N and corn stover in dry farmland.

Uptake and Recovery of Soil Nitrogen by Bryophytes and Vascular Plants in an Alpine Meadow

Due to their particular physiology and life history traits, bryophytes are critical in regulating biogeochemical cycles and functions in alpine ecosystem. Hence, it is crucial to investigate their nutrient utilization strategies in comparison with vascular plants and understand their responses to the variation of growing season caused by climate change. Firstly, this study testified whether or not bryophytes can absorb nitrogen(N) directly from soil through spiking three chemical forms of 15N stable isotope tracer. Secondly, with stronger ability of carbohydrates assimilation and photosynthesis, it is supposed that N utilization efficiency of vascular plants is significantly higher than that of bryophytes. However, the recovery of soil N by bryophytes can still compete with vascular plants due to their greater phytomass. Thirdly, resource acquisition may be varied from the change of growing season, during which N pulse can be manipulated with 15N tracer addition at different time. Both of bryophytes and vascular plants contain more N in a longer growing season, and prefer inorganic over organic N. Bryophytes assimilate more NH4+ than NO3– and amino acid, which can be indicated from the greater shoot excess 15N of bryophytes. However, vascular plants prefer to absorb NO3– for their developed root systems and vascular tissue. Concerning the uptake of three forms N by bryophytes, there is significant difference between two manipulated lengths of growing season. Furthermore, the capacity of bryophytes to tolerate N-pollution may be lower than currently appreciated, which indicates the effect of climate change on asynchronous variation of soil N pools with plant requirements.

Fertilizer ^(15)N Accumulation, Recovery and Distribution in Cotton Plant as Affected by N Rate and Split

N fertilization of 300 kg N ha-1 is normally applied to cotton crops in three splits： pre-plant application （PPA, 30%）, first bloom application （FBA, 40%） and peak bloom application （PBA, 30%） in the Yangtze River Valley China. However, low fertilizer N plant recovery （NPR） （30-35%） causes problems such as cotton yield stagnation even in higher N rate, low profit margin of cotton production and fertilizer release to the environment. Therefore, it is questioned： Are these three splits the same significance to cotton N uptake and distribution？ An outdoor pot trial was conducted with five N rates and 15 N labeled urea to determine the recovery and distribution of 15N from different splits in cotton （Gossypium hirsutum L. cv. Huazamian H318） plant. The results showed that, cotton plant absorbed fertilizer 15N during the whole growing period, the majority during flowering for 18-20 d regardless of N rates （150-600 kg ha-1）. Fertilizer 15N proportion to the total N accumulated in cotton plant increased with N rates, and it was the highest in reproductive organs （88% averaged across N rates） among all the plant parts. FBA had the highest NPR （70%）, the lowest fertilizer N lose （FNL, 19%）, and the highest contribution to the fertilizer 15N proportion to the total N （46%） in cotton plant, whereas PPA had the reverse effect. It suggests that FBA should be the most important split for N absorption and yield formation comparatively and allocating more fertilizer N for late application from PPA should improve the benefit from fertilizer.

Fate of ^(15)N Labeled Nitrate and Ammonium Salts Added to an Alpine Meadow in the Qinghai-Xizang Plateau, China

To understand the dynamics of added nitrogen (N) in alpine meadow and the role of alpine plants and soil microorganisms in the retention of deposited N, the fate of 15 N labeled nitrate and ammonium salts was determined in an alpine meadow for two months. Two weeks after 15 N application, total recovery of 15 N from NO - 3_ 15 N was 73.5% while it was 78% from NH + 4_ 15 N. More 15 N was recovered in plants than in soil organic matter or in microbial biomass, irrespective of forms of N added. After one month, 70.6% of added NO - 3_ 15 N and 57.4% of NH + 4_ 15 N were recovered in soils and plants. 15 N recovered in soil organic matter decreased greatly while that recovered in plants varied little, irrespective of the form N. Compared with the results of two weeks after 15 N application, more NO - 3_ 15 N than NH + 4_ 15 N was recovered in microbial biomass. Total recovery was 58.4% (six weeks) and 67% (eight weeks) from NO - 3_ 15 N, and 43.1% and 49% from NH + 4_ 15 N, respectively. Both plants and soil microorganism recovered more NO - 3_ 15 N than NH + 4_ 15 N. But plants recovered more 15 N than soil microorganisms. During the whole experiment plants retained more NO - 3_N and 15 N than soil microorganisms while 15 N recovered in inorganic N pool did not exceed 1% due to lower amount of inorganic N. This indicates that plants play more important roles in the retention of deposited N although microbial biomass can be an important sink for deposited N in early days after N application.

Mineral Coated Fertilizer Effect on Nitrogen-Use Efficiency and Yield of Wheat

A field experiment with winter wheat （Triticum aestivurn L.） was conducted on a silt loam calcaric endorusti-ustic Cambosols derived from the Yellow River alluvial deposits in Henan, China, from 2001 to 2002 to evaluate N recovery and agronomic performance of different mineral coated fertilizers （MiCFs） compared to normal urea used in wheat cropping systems under field conditions. Five treatments, including CK （check, no N fertilizer）, urea and three different MiCFs at an equivalent N application rate were established in a randomized complete block design. N release from MiCFs in soil was more synchronous with the N requirement of wheat throughout the growth stages than that from urea, with grain yield of the MiCF treatments significantly higher （P 〈 0.05） than that of the treatment urea. Correspondingly, the N recovery rate was greater for all MiCFs compared to urea, increasing from 32.8% up to 50.1%. Due to its high recovery and low cost, use of the mineral coated N fertilizers was recommended instead of the polymer coated N fertilizers.

Contributions of Different N Sources to Crop N Nutrition in a Chinese Rice Field

N availability is one of the most important factors limiting crop yield enhancement.The recovery of applications of 15 N-labeled fertilizer and crop residues in a rice-wheat cropping system was determined for up to 6 consecutive growing seasons.The crop residues from the previous season were either incorporated or removed as two different treatments.Our results showed that 16.55%-17.79% (17.17% on average) of the fertilizer N was recovered in the crop during the first growing season,suggesting that more than 80% of crop N was not directly from the N fertilizer.When 15 N-labeled residues were applied,12.01% was recovered in the crop in the first growing season.The average recoveries of fertilizer N and crop residue N in the soil after the first growing season were 33.46% and 85.64%,respectively.N from soil organic matter contributed approximately 83% of the N in the crop when 15 N fertilizer was applied or 88% when crop residues were applied.There was a larger difference in the total 15 N recovery in plant and soil between N applications in the forms of fertilizer and crop residues.Incorporation of crop residues following the 15 N fertilizer application did not significantly promote 15 N recovery in the crop or soil.On average,only additional 1.94% of N for the fertilizer-applied field or 5.97% of N for the crop residue-applied field was recovered by the crops during the 2nd and 3rd growing seasons.The total recoveries of 15 N in crop and soil were approximately 64.38% for the fertilizer-applied field after 6 growing seasons and 79.11% for the crop residue-applied field after 5 growing seasons.Although fertilizer N appeared to be more readily available to crops than crop residue N,crop residue N replenished soil N pool,especially N from soil organic matter,much more than fertilizer N.Therefore,crop residue N was a better source for sustaining soil organic matter.Our results suggested that the long-term effect of fertilizer or crop residues on N recovery were different in the crop and soil.However,there was little difference between the practices of crop residue incorporation and residue removal following the N fertilizer application.

The fate of fertilizer N applied to cotton in relation to irrigation methods and N dosage in arid area

Quantitative information on the fate and efficiency of nitrogen （N） fertilizer applied to coarse textured calcareous soils in arid farming systems is scarce but, as systems intensify, is essential to support sustainable ag- ronomic management decisions. A mesh house study was undertaken to trace the fate of N fertilizer applied to cotton （Gossypium hirsutum L. cv., Huiyuan701） growing on a reconstructed profile （0-100 cm） of a calcareous （〉15% CaCQ） sandy loam soil. Two irrigation methods （drip irrigation, DI; and furrow irrigation, FI） and four N ap- plication rates （0, 240, 360 and 480 kg/hm2, abbreviated as No, N240, N360, and N480, respectively） were applied. 15N-labelled urea fertilizer was applied in a split application. DI enhanced the biomass of whole plant and all parts of the plant, except for root; more fertilizer N was taken up and mostly stored in vegetative parts; N utilization efficiency （NUE） was significantly greater than in FI. N utilization efficiency （NUE） decreased from 52.59% in N240 to 36.44% in N480. N residue in soil and plant N uptake increased with increased N dosage, but recovery rate decreased consis- tently both in DI and Fl. Plant N uptake and soil N residue were greater in DI than in FI. N residue mainly stayed within 0-40 cm depth in DI but within 40-80 cm depth in Ft. FI showed 17.89% of N leached out, but no N leaching occurred in DI. N recovery rate in the soil-plant system was 75.82% in DI, which was markedly greater than the 55.97% in FI. DI exhibited greater NUE, greater residual N in the soil profile and therefore greater N recovery rate than in FI; also, N distribution in soil profile shallowed in DI, resulting in a reduced risk of N leaching compared to FI; and enhanced shoot growth and reduced root growth in DI is beneficial for more economic yield formation. Com- pared to furrow irrigation, drip irrigation is an irrigation method where N movement favors the prevention of N from being lost in the plant-soil system and benefits a more efficient use of N.

Reducing nitrogen application with dense planting increases nitrogen use efficiency by maintaining root growth in a double-rice cropping system

Rational nitrogen(N) application can greatly increase rice(Oryza sativa L.) yield. However, excessive N input can lead not only to low N use efficiency(NUE) but also to severe environmental pollution.Reducing N application rate with a higher planting density(RNHD) is recommended to maintain rice yield and improve NUE. The effects of RNHD on fertilizer N fate and rice root growth traits remain unclear. We accordingly conducted a two-year field experiment to investigate the influence of RNHD on rice yield, fertilizer 15N fate, and root growth in a double-rice cropping system in China. In comparison with the conventional practice of high N application with sparse planting, RNHD resulted in similar yield and biomass production as well as plant N uptake. RNHD increased agronomic NUEs by 23.3%–31.9%(P < 0.05) and N recovery efficiency by 17.4%–24.1%(P < 0.05). RNHD increased fertilizer 15N recovery rate by 14.5%–34.7%(P < 0.05), but reduced15 N retention rate by 9.2%–12.0%(P < 0.05). Although a reduced N rate led to significantly reduced root length, surface area, volume, and biomass, these root traits were significantly increased by higher planting density. RNHD did not affect these root morphological traits and reduced activities of nitrate reductase(NR) and glutamine synthetase(GS) only at tillering stage. Plant N uptake was significantly positively correlated with these root traits, but not correlated with NR and GS activities. Together, these findings show that reducing N application with dense planting can lead to high plant N uptake by maintaining rice root growth and thus increase NUE.

Dynamics of Nutrient Accumulation in Maize Plants Under Different Water and N Supply Conditions

The dynamics of accumulations of plant dry matter, nutrient uptake and N fertilizer recovery were studied with different water and N supply, using summer maize (Zea mays L. var. Shandan9) as an indicator crop. The total dry matter (including roots) and N, P, K uptake amounts were continuously increased with plant growth, and their accumulations with time during plant-growing period were shaped in S curves that could be described by exponential regression equations. Differentiating the regression equations fitting the curves over time for first derivatives, the momentary rate was obtained of the dry matter and nutrient uptake. Results show that the dry matter and the nutrient uptake were not in the same rate at all time, but changed from one time to another. Usually, the rate increased rapidly at early stages, and gradually decreased after reaching their peak. Of N, P and K, the uptake rate of N and K was higher, and their increase and decrease were both fast while P was reversed. The time of the maximum absorptive rate appeared earlier for K, followed by N, and then by P. In any case, the maximum nutrient uptake rate appeared earlier than did the dry matter. The momentary N recovery rate was similar in trend to those of dry matter and N uptake, and its maximum recovery rate occurred almost at the same time as its maximum uptake rate. Supplemental irrigation raised the cumulative and momentary rates of N. Although water and N supplies increased dry matter and nutrient uptake rates, they did not alter their changing trends during the plant-growing period.

Optimize nitrogen fertilization location in root-growing zone to increase grain yield and nitrogen use efficiency of transplanted rice in subtropical China

The optimized nitrogen fertilization location differs in different rice-growing regions. We optimized nitrogen deep-point application in root-growing zone（NARZ） for transplanted rice in subtropical China. Field plot experiments were conducted over two years（2014–2015） in a double-rice cropping system to evaluate the effects of nitrogen（N） fertilizer location on grain yield and N use efficiency（NUE）. Four different nitrogen deep-point application methods（DN） were compared with traditional broadcast application（BN） using granular urea. The results showed that grain yield, recovery efficiency of N（REN）, agronomic efficiency of N（AEN）, and partial factor productivity of N（PFP_N） significantly increased 10.3–63.4, 13.7–56.7, 24.7–201.9 and 10.2–63.4%, respectively, in DN treatment compared to BN, respectively. We also find that DN treatments increased grain yield as well as grain N content, and thus grain quality, in comparison with conventional BN treatment. Correlation analysis indicated that significant improvement in grain yield and NUE mainly resulted from increases in productive panicle number and grain N content. In our proposed NARZ method, granular urea should be placed 0 to 5 cm around the rice seeding at a 12-cm depth druing rice transplanting. In NARZ, balanced application of N, P and K further improved grain yield and NUE over treatments with a single N deep-point application. High N uptake by the rice plant did not cause significant soil fertility depletion, demonstrating that this method could guarantee sustainable rice production.

Araucaria cunninghamii Seedling Response to Different Forms and Rates of ^(15)N-Labelled Fertiliser

Nitrogenous fertilisers are under consideration for promoting the growth of nursery-reared hoop pine （Araucaria cunninghamii Alton ex A. Cunn） seedlings in the establishment phase of second rotation （2R） plantations. Using ^15N- labelled fertilisers, we investigated the effect of different forms （ammonium sulphate, ammonium nitrate, potassium nitrate and urea） and rates of application （0, 150 and 300 mg N kg^-1 dried soil） of fertilisers on the growth, ^15N recovery and carbon isotope composition （δ^13C） of hoop pine seedlings in a 12-month glasshouse trial in southeast Queensland, Australia. The ^15N-labelled fertilisers were applied to nursery-reared hoop pine seedlings, which were then grown in pots, containing ca. 1.2 kg dried soil, under well watered conditions for 12 months. Four seedlings from each treatment were harvested at 4-month intervals, divided into roots, stem and foliage, with a further subdivision for new and old foliage, and then analysed for ^15N, total N, δ^13C and total C. There was no significant response in the seedling growth to the form or rate of application of nitrogen （N） fertiliser within the 12-month period, indicating that the seedlings did not experience N deficiency when grown on second rotation hoop pine soils. While the combined ^15N recovery from soil and plant remained at around 70% throughout the experiment, the proportion of ^15N recovered from the plants increasing steadily over time. Nitrate containing fertilisers at 150 mg N kg^-1 soil gradually increased seedling foliage δ^13C over the 12-month period, indicating an increase in seedling water use efficiency.

In-Season Root-Zone Nitrogen Management Strategies for Improving Nitrogen Use Efficiency in High-Yielding Maize Production in China

Many recently developed N management strategies have been extremely successful in improving N use efficiency. How- ever, attempts to further increase grain yields have had limited success. Field experiments were conducted in 2007 and 2008 at four sites to evaluate the effect of an in-season root-zone N management strategy on maize （Zea mays L.）. According to the in-season root-zone N management, the optimal N rate （ONR） was determined by subtracting measured soil mineral N （NHa＋-N and NO3-N） in the root zone from N target values. Other treatments included a control without N fertilization, 70% of ONR~ 130% of ONR, and recommended N rate （RNR） by agronomists in China that have been shown to approach maize yield potentials. Although apparent N recovery for the ONR treatment was significantly higher than that under RNR in 2007, grain yield declined from 13.3 to 11.0 Mg ha-1 because of an underestimation of N uptake. In 2008, N target values were adjusted to match crop uptake, and N fertilization rates were reduced from 450 kg N ha-1 for RNR to 225 to 265 kg N ha 1 for ONR. High maize yields were maintained at 12.6 to 13.5 Mg ha 1 which were twice the yield from typical farmers＇ practice. As a result, apparent N recovery increased from 29% to 66%, and estimated N losses decreased significantly for the ONR treatment compared to the RNR treatment. In conclusion, the in-season root-zone N management approach was able to achieve high yields, high NUE and low N losses.

Optimization of nitrogen fertilizer rate under integrated rice management in a hilly area of Southwest China

China has the world’s highest nitrogen(N)application rate,and the lowest N use efficiency(NUE).With the crop yield increasing,serious N pollution is also caused.An in-situ field experiment(2011–2015)was conducted to examine the effects of three N levels,0(i.e.,no fertilizer N addition to soil),120,and 180 kg N ha-1,using integrated rice management(IRM).We investigated rice yield,aboveground N uptake,and soil surface N budget in a hilly region of Southwest China.Compared to traditional rice management(TRM),IRM integrated raised beds,plastic mulch,furrow irrigation,and triangular transplanting,which significantly improved rice grain yield,straw biomass,aboveground N uptake,and NUE.Integrated rice management significantly improved 15N recovery efficiency(by 10%)and significantly reduced the ratio of potential15N loss(by 8%–12%).Among all treatments,the 120 kg N ha-1 level under IRM achieved the highest 15N recovery efficiency(32%)and 15N residual efficiency(29%),with the lowest 15N loss ratio(39%).After rice harvest,the residual N fertilizer did not achieve a full replenishment of soil N consumption,as the replenishing effect was insufficient(ranging from-31 to-49 kg N ha-1).Furthermore,soil surface N budget showed a surplus(69–146 kg N ha-1)under all treatments,and the N surplus was lower under IRM than TRM.These results indicate IRM as a reliable and stable method for high rice yield and high NUE,while exerting a minor risk of N loss.In the hilly area of Southwest China,the optimized N fertilizer application rate under IRM was found to be 100–150 kg N ha-1.

Growth and nitrogen status of cotton(Gossypium hirsutum L.)under salt stress revealed using^(15)N-labeled fertilizer

Salt stress is a vital factor limiting nitrogen uptake and cotton growth in arid regions.The mechanisms underlying salt stress tolerance in cotton plants under high soil salinity have not been fully elucidated.Therefore,the aim of this study was to examine the proportion and mechanism of cotton nitrogen uptake under salt stress using the^(15)N isotope labeling technique.Cotton plants were grown in four undisturbed saline soils(1,3,6 and 9 dS m^(-1)),and the experiment was designed using the ENVIRO-GRO(E-G)model.The results showed that the dry matter of roots,stems and leaves of the cotton parts in slightly saline soil(C2,3 dS m^(-1))was not significant compared with the non-saline soil(C1,1 dS m^(-1)).The cotton fruit grown in low-salinity soil(C2,3 dS m^(-1))had significantly higher dry matter than that grown in the other treatments,implying that cotton plants grown in 3 dS m^(-1)soil have the best nitrogen uptake and salt tolerance.Cotton plants grown in weakly(C3,6 dS m^(-1))and moderately(C4,9 dS m^(-1))saline soils exhibited premature senescence.The distribution of total nitrogen and nitrate content in cotton was the best explanatory variable of total^(15)N recovery,of which cotton^(15)N recovery was between 26.1%and 47.2%,and soil^(15)N recovery was between 7.7%and 14.9%.Our findings provide guidance for further exploitation and utilization of saline soil resources and sustainable development of the agricultural soil ecosystem in arid regions.