Leaching of nitrogen forms from controlled‐release nitrogen fertilizers

Abstract

Application of soluble forms of nitrogen (N) fertilizers to sandy soils may cause leaching of nitrate N (NO₃‐N) resulting in contamination of groundwater. The leaching loss of N may be reduced to a certain extent by the use of controlled‐release N formulations. A leaching column study was conducted to evaluate the leaching of urea, ammonium N (NH₄‐N), and NO₃‐N forms from selected urea‐based controlled‐release formulations (Meister, Osmocote, and Poly‐S) and uncoated urea under eight cycles of intermittent leaching and dry conditions. Following leaching of 1,760 mL of water (equivalent to 40 cm rainfall) through the soil columns, the recovery of total N (sum of all forms) in the leachate accounted for 28, 12, 6, or 5% of the total N applied as urea, Poly‐S, Meister, and Osmocote, respectively. Loss of urea‐N from all fertilizer sources was pronounced during the initial leaching events (with the exception of Meister). Cumulative leaching of urea‐N was 10% for uncoated urea while <1.7% for the controlled‐release formulations. Cumulative leaching of NH₄‐N was 6.2% for uncoated urea while <0.5% for the controlled‐release formulations. Cumulative leaching loss of NO₃‐N was 3.78% for Osmocote, 4.6% for Meister, 10.4% for urea, and 10.5% for Poly‐S. This study demonstrates a significant reduction in leaching of N forms from controlled‐release formulations as compared to that from the soluble form.     

Relationships between nitrogen rate,plant nitrogen concentration,yield and residual soil nitrate‐nitrogen in silage corn

Abstract

Nitrogen (N) fertilizer is a key factor of yield increase but also an environmental pollution hazard. The sustainable agriculture system should have an acceptable level of productivity and profitability and an adequate environmental protection. The objectives of this study were to determine the relationships between N rate, DM yield, plant N concentration (NC) and residual soil nitrate‐nitrogen in order to improve the predicted N rate in corn (Zea mays L.) silage. The experiment was conducted over a period of three years in the province of Quebec on three soil series in a continuous corn crop sequence. Treatments consisted of six rates of N: O, 40, 80, 120, 160, and 200 kg N ha^‐1 as ammonium nitrate applied at planting: broadcast and side banded. Four optimum N rates were calculated using different models: (i) economic rate base on fertilizer and corn price using the quadratic model (E); (ii) economic rate based on fertilizer and corn price using the quadratic‐plus‐plateau model (QP); (iii) critical rate based on linear‐plus‐plateau model (P); (iv) lower than maximum rate (L) corresponding to 95% of maximum yield. The optimum plant NC at all growing stages and the N uptake at harvest were calculated depending on these N rates and yields.

The NC of whole plant at 8‐leaf stage (25–30 cm plant height) of ear leaf at tasselling and of whole plant at harvest stage, the N rate, the N uptake at harvest and the DM yield were all significantly intercorrelated and affected by soils and years, but not affected by N fertilizer application method. The DM yield was linearly and significantly related to NC of whole plant at 8‐leaf stage (rv = 0.932**). At this stage, the average NC corresponding to the optimum N rate and yield was of 3.71, 3.68, and 3.66% as calculated with E, L, and P model, respectively. Our data suggest that the NC of whole plant at 8‐leaf stage may be used to evaluate the N nutrition status of plant and the required optimum N fertilizer rate. The NC of ear leaf at tassel stage was also significantly correlated to corn yield (r = 0.994**). It may be used as an indicator to evaluate the near‐optimum N rate in the subsequent years.

The N uptake by whole above‐ground plant at harvest was quadratically related to corn yield. Data show that at high fertilizer N rate, the N uptake still increased without significantly increasing yield. The N uptake was of 176.5, 163.0, and 155.0 kg N ha^‐1 using the E, L and P rates of 146, 126, and 115 kg N applied ha^‐1, respectively. The optimum N rate and yield were affected by soil type and year, but not by the method of N fertilizer application. The yield increased rapidly up to a N rate of about 120 kg N ha^‐1 and then quite slightly to a maximum N rate of 192 kg N ha^‐1. The optimum N rate was of 115 and 126 kg N ha^‐1 using the P and L model respectively and as high as 146.8 kg N ha^‐1 using the E model. The L model, using a much smaller N rate, gave a reasonably high yield compared to E rate (12.2 and 12.5 Mg ha^‐1, respectively). The data show that a relatively much lower N rate than maximum did not proportionally diminish the yield. Thus, for a difference of 40.4% between maximum N rate and P rate a difference of only 7.4% in yield was observed. Using the L model the differences in rate and yield were of 34.4% and 4.7%, respectively. The QP model gave no significant difference compared to E model.

At harvest the residual soil NO₃‐N increased significantly with increasing N fertilizer rate in whole of the 100 cm soil profile, but mainly in the top 40 cm soil layer. The total NO₃‐N found in 0–100 cm profile at rate of 0, 120 and 200 kg applied N ha^‐1 at planting was as high as 33.7, 60.5, and 74.5 kg N ha^‐1 respectively in a light soil and 37.5, 97.5, and 145.5 kg N ha^‐1 in a heavy clay soil. The difference in NO₃‐N content in the 60–100 cm layer between different applied N rate suggests that at harvest, part of fertilizer N applied at planting was already leached below the 100 cm soil layer. Results, thus, show that reasonably high corn yields can be obtained using more adequate N fertilizer rates which avoid the overfertilization and are likely to reduce the air and ground water pollution.     

Cover crop nitrogen availability to conventional and no‐till corn: Soil mineral nitrogen,corn nitrogen status,and corn yield

Abstract

Understanding seasonal soil nitrogen (N) availability patterns is necessary to assess corn (Zea mays L.) N needs following winter cover cropping. Therefore, a field study was initiated to track N availability for corn in conventional and no‐till systems and to determine the accuracy of several methods for assessing and predicting N availability for corn grown in cover crop systems. The experimental design was a systematic split‐split plot with fallow, hairy vetch (Vicia villosa Roth), rye (Secale cereale L.), wheat (Triticum aestivum L.), rye+hairy vetch, and wheat+hairy vetch established as main plots and managed for conventional till and no‐till corn (split plots) to provide a range of soil N availability. The split‐split plot treatment was sidedressed with fertilizer N to give five N rates ranging from 0–300 kg N ha^‐1 in 75 kg N ha^‐1 increments. Soil and corn were sampled throughout the growing season in the 0 kg N ha^‐1 check plots and corn grain yields were determined in all plots. Plant‐available N was greater following cover crops that contained hairy vetch, but tillage had no consistent affect on N availability. Corn grain yields were higher following hairy vetch with or without supplemental fertilizer N and averaged 11.6 Mg ha^‐1 and 9.9 Mg ha^‐1 following cover crops with and without hairy vetch, respectively. All cover crop by tillage treatment combinations responded to fertilizer N rate both years, but the presence of hairy vetch seldom reduced predicted fertilizer N need. Instead, hairy vetch in monoculture or biculture seemed to add to corn yield potential by an average of about 1.7 Mg ha^‐1 (averaged over fertilizer N rates). Cover crop N contributions to corn varied considerably, likely due to cover crop N content and C:N ratio, residue management, climate, soil type, and the method used to assess and assign an N credit. The pre‐sidedress soil nitrate test (PSNT) accurately predicted fertilizer N responsive and N nonresponsive cover crop‐corn systems, but inorganic soil N concentrations within the PSNT critical inorganic soil N concentration range were not detected in this study.     

Effects of supplemental nitrogen on nitrogen‐assimilation enzymes,free amino nitrogen,soluble sugars,and crude protein of rice

Abstract

An upland rice variety IAC‐47 was grown in a greenhouse to determine the effect of foliar nitrogen (N) supplementation during grain development on the activity of the N assimilation enzymes, nitrate reductase (NR) and glutamine synthetase (GS), on free amino‐N content and leaf soluble sugars, and on grain crude protein content. At 10 and 20 days after anthesis (DAA), the leaves were fertilized with a liquid fertilizer containing 32% N as 12.8% urea, 9.6% ammonium (NH₄), and 9.6% nitrate (NO₃) in increasing rates corresponding to 0,20+20, 40+40, and 60+60 kg N ha^‐1. Leaves were collected twice (at 12 DAA and 14 DAA for GS activity, sugar and amino‐N content, and at 11 and 13 DAA for NRA) after each application of leaf N. The late foliar application of N increased significantly grain crude protein without a corresponding decrease in grain weight. The NR activity (NRA) increased after the foliar application of N. In the flag leaf, 60+60 kg N ha^‐1 (21 DAA) resulted in higher NRA (20x over the control), while GS activity was smaller than the control. At 22 DAA there was an increase in GS activity in the flag leaf at 20+20 N level. However, the GS activity decreased as applied N levels increased. Also at the 20+20 level, there were increases in free amino‐N in the flag leaf and second leaf at the final harvest. Throughout the experiment, plants at the 60+60 N level had the lowest levels of soluble sugars. Increases in crude protein were highest at 40+40 N level (27.9%), followed by 60+60 (18.7%).     

Determination of dissolved organic nitrogen using persulfate oxidation and conductimetric quantification of nitrate‐nitrogen

Abstract

Nitrogen (N) in forest soil extracts and surface waters may be dominantly in organic compounds as dissolved organic nitrogen (DON). Due to various difficulties associated with measuring total N (as TKN) by the Rjeldahl digest, this important vehicle for nutrient movement is rarely monitored. By coupling two relatively new methods and optimizing them for use in soil studies, we developed an alternative method for measuring DON. Analysis of pure compounds and field samples shows that persulfate oxidation combined with conductimetric quantification of nitrate (NO₃) provides a highly accurate measure of dissolved N content. With relatively inexpensive equipment and reagents, a single technician can digest and assay over a hundred samples a day. This rapid, simple, and accurate assay may make it possible to routinely monitor DON where it had previously been impractical. This in turn could substantially enhance understanding about the form and quantity of N involved in nutrient fluxes.     

Determination of nitrogen by microdiffusion in mason jars: III. Nitrogen and nitrogen‐15 in Kjeldahl digests

Abstract

Diffusion methods for quantitative determination and isotope‐ratio analysis of inorganic N in soil extracts were modified for use with Kjeldahl digests. The digest was diluted to 25 mL with deionized water, and an aliquot (to 6 mL) was transferred in a shell vial (17 mm dia., 60 mm long) to a 473‐mL (1‐pint) wide‐mouth Mason jar containing 15 mL of 8 M NaOH. The NH₃ liberated by overturning the vial inside the sealed jar was collected for 48 h at room temperature (24 h with orbital shaking) in 3 mL of boric acid‐indicator solution in a Petri dish, or in an acidified glass‐fiber disk, suspended from the Mason‐jar lid. Determinations of N and ¹⁵N by diffusion were in close agreement with analyses using conventional steam‐distillation and concentration techniques.     

Nitrate‐nitrogen determination—a critical review

Abstract

This paper reviews the published methods of nitrate‐nitrogen (NO₃‐N) determination with the objective to assess their applicability to soil and plant tissue anarysis. The methods are separated into three categories on the basis of the analytical approach utilized for NO₃‐N determination. Strengths and weaknesses of the methods are discussed. The first analytical approach utilitizes direct measurement of NO₃‐N by the following methods: (a) colorimetric (after a color producing reaction with NO₃‐N), (b) potentiometric, (c) absorption of UV radiation by NO₃‐N in a complex matrix, (d) transnitration of salicylic acid, and (e) chromatographic (separation and measurement of NO₃‐N) methods. The second approach is based on the reduction of NO₃‐N to nitrite‐nitrogen (NO₂‐N), ammonium‐nitrogen (NH₄‐N), or nitric oxide and measurement of the reduction product. When NO₃‐N is reduced to NO₂‐N, the measurement may be achieved by (a) colorimetric, (b) fluorimetric, (c) coulometric, and (d) catalytic kinetic methods. When NO₃‐N is reduced to NH₄‐N, the measurement is done by (a) colorimetric (after a color producing reaction with NH₄), (b) potentiometric, (c) steam distillation, and (d) gas diffussion conductimetric methods. A chemiluminescence detection method is utilized when NO₃‐N is reduced to nitric oxide. The third approach determines NO₃‐N concentration by measuring the change in the concentration of the chemical species that react with NO₃‐N and form a complex.     

Improvement of the nitrogen uptake induced by titanium (iv) leaf supply in nitrogen‐stressed pepper seedlings

The beneficial effect of titanium (Ti) on plant metabolism can result in more profitable use of fertilizer applied to a crop. A crop chamber experiment with paprika pepper (Capsicum annuum L., cv. Bunejo) seedlings under differential nitrogen (N) concentration levels in a nutrient solution (100% N, 75% N, 50% N, and 25% N) was performed. A third of the seedlings growing under each N support level remained Ti‐untreated and were used as the reference. Another third of the seedling received one and two 0.042 mM Ti(TV) ascorbate, pH 6.0, leaf spray treatments, respectively. The biomass production of the Ti‐untreated plants was only affected by the N supply of 50% or less. The Ti(IV) leaf spray treatments produced a biomass production greater than that of the corresponding reference plants, and both the 100% N+Ti and 75% N+Ti treatments had the highest biomass production. Seedlings receiving 50% N+Ti had a level of biomass production similar to that for the 100% N without Ti reference plants. In the same way, the 25% N+Ti treatment resulted in a plant fresh weight greater than that for the Ti‐untreated reference plants, although their biomass yields were not significantly lower than that for the corresponding 100% N and 75% N Ti‐untreated reference plants. Only the 50% N and 25% N Ti‐untreated plants had definite total‐N and nitrate‐nitrogen (NO₃‐N) unbalances as compared to the other N rate‐Ti treatments.     

Salinity effects on nitrogen metabolism in plants – focusing on the activities of nitrogen metabolizing enzymes: A review

Nitrogen (N) metabolism is of great economic importance because it provides proteins and nucleic acids which in turn control many cellular activities in plants. Salinity affects different steps of N metabolism including N uptake, NO₃^? reduction, and NH₄⁺ assimilation, leading to a severe decline in crop yield. Major mechanisms of salinity effects on N metabolism are salinity-induced reductions in water availability and absorption, disruption of root membrane integrity, an inhibition of NO₃^? uptake by Cl^?, low NO₃^? loading into root xylem, alteration in the activities of N assimilating enzymes, decrease in transpiration, and reduction in relative growth rate which results in a lower N demand. However, the effects of salinity on N metabolism are multifaceted and may vary depending on many plant and soil factors. The present review deals with salinity effects on N metabolism in plants, emphasizing on the activities of N metabolizing enzymes in a saline environment.     

Uptake of ammonium‐nitrogen by blueberry plants

Blueberry plants (Vaccinium ashei Reade cv. Tifblue) and Citrus natsudaidai Hayata were compared in terms of their ability to regulate the uptake of ammonium‐nitrogen (NH₄‐N). Plants of both species were grown in N‐free nutrient solutions for three days and then transferred to nutrient solutions that contained various concentrations of NH4‐N. Blueberry plants showed increases in rates of uptake of NH₄‐N 8 to 24 h after application of NH₄‐N. At concentrations of NH₄‐N above 200 (μM, uptake rates decreased to the initial value 24 h after application of NH₄‐N and then increased. By contrast, seedlings of Citrus natsudaidai showed constant rates of uptake of NH₄‐N during the experiment. These results indicate that blueberry plants are able to repress the uptake of NH₄‐N periodically when they are exposed to high concentrations of external NH₄‐N, but not seedlings of Citrus natsudaidai.     

Winter wheat fertilizing using nitrogen–sulphur fertilizer

Precise field experiments were established on two sites with winter wheat under different soil-climatic conditions in the Czech Republic. Four treatments were fertilized with same dose of nitrogen (200 kg N ha^?1) and increasing dose of sulphur (0, 10, 20 and 40 kg S ha^?1) using nitrogen–sulphur (N–S) fertilizer with calcium sulphate form. Soil and plant aboveground biomass samples were taken in the stages of development BBCH 26–28; 30–32; 37–39; 49–51. The winter wheat grain yield ranged between 7.20 and 10.86 t ha^?1 and had an increasing trend with increasing sulphur dose. Although the differences were usually not statistically significant, there were found increasing tendencies of bioavailable sulphur content in soil with increasing S split doses. Soil S content decreased with time probably due to plant uptake. Sulphur dose did not influence the S content in plant aboveground biomass. The total S contents in grain after harvest ranged between 0.09% and 0.14% and were not significantly influenced with the fertilizing treatment. The same statement is valid for the S content in straw, which ranged between 0.03% and 0.11%. Both, S content in winter wheat seeds and straw were strongly influenced by the site conditions.     

Long-term fertilizer nitrogen management—Soil health conundrum

<正>The mental model that fertilizer nitrogen (N) acts as a replacement for N mineralized from soil organic matter (SOM) needs to be revisited. Soil organic matter, the storehouse of N in soil, is one of the most important indicators of soil health. It supplies more N to crop plants than the current-year fertilizer N even when applied at high rates. Limited research shows that the application of fertilizer N above the optimum levels on a long-term basis deteriorates soil health by mineraliz...     

Evaluation of methods for nitrogen‐15 analysis of inorganic nitrogen in soil extracts: I. Steam‐distillation methods

Abstract

A study was conducted to evaluate conventional steam‐distillation techniques for N‐isotope analysis of inorganic forms of N in soil extracts. Extracts obtained with 2 M KCl from 10 diverse soils were treated with: (i) (¹⁵NH₄)₂SO₄ and KNO₃, (ii) (NH₄)₂SO₄ and K¹⁵NO₃, or (iii) KNO₃and Na¹⁵NO₂. Steam distillations were performed sequentially to determine NH₄ ⁺‐N and NO₃ ^‐‐N, and were also carried out to determine (NO₃ ^‐ + NO₂ ^‐)‐N or (NH₄ ⁺ + NO₃ ^‐ + NO₂ ^‐)‐N; a pretreatment with sulfamic acid was used to determine NO₃ ^‐‐N in the presence of NO₂ ^‐‐N. Recovery of added N ranged from 95 to 102%. Significant isotopic contamination was observed in sequential distillation of unlabeled NO₃ ^‐‐N following labeled NH₄ ⁺‐N; otherwise, analyses for ¹⁵N were usually within 1% of the values calculated by isotope‐dilution equations.     

Influence of nitrate‐nitrogen and ammonium‐nitrogen on calcium (45Ca2+) absorption in sorghum root tips

Abstract

Root‐tip, 1‐cm of Sorghum bicolor (L.) Moench cv SC283, SC574, GP‐10, and Funk G522DR were exposed to calcium (⁴⁵Ca²⁺) at pH 5.5 for 2‐hr in the presence of nitrate‐nitrogen (NO3^?‐N) or ammonium‐nitrogen (NH4+‐N). Nitrate (0.1 mM) induced significantly increased ⁴⁵Ca uptake in Funk G522DR, SC283, and GP‐10 while 0.01 mM NO₃ ^?‐N induced significantly increased ⁴⁵Ca'uptake in SC574, but ⁴⁵Ca absorption was significantly decreased at 1 mM NO₃—N. In the presence of the NH4⁺ ion, ⁴⁵Ca uptake was increased up to 8X that of the NH₄ ⁺‐N untreated roots. When ammonium chloride (NH₄CI) was used, the Cl^? tended to induce an increased ⁴⁵Ca uptake. Cultivar variation was present.     

Leaf nitrogen determination using non-destructive techniques–A review

The optimisation of plant nitrogen-use-efficiency (NUE) has a direct impact on increasing crop production by optimising use of nitrogen fertiliser. Moreover, it protects environment from negative effects of nitrate leaching and nitrous oxide production. Accordingly, nitrogen (N) management in agriculture systems has been major focus of many researchers. Improvement of NUE can be achieved through several methods including more accurate measurement of foliar N contents of crops during different growth phases. There are two types of methods to diagnose foliar N status: destructive and non-destructive. Destructive methods are expensive and time-consuming, as they require tissue sampling and subsequent laboratory analysis. Thus, many farmers find destructive methods to be less attractive. Non-destructive methods are rapid and less expensive but are usually less accurate. Accordingly, improving the accuracy of non-destructive N estimations has become a common goal of many researchers, and various methods varying in complexity and optimality have been proposed for this purpose. This paper reviews various commonly used non-destructive methods for estimating foliar N status of plants.     

A quick test procedure for soil nitrate‐nitrogen

Abstract

A procedure for extraction and measurement of nitrate‐nitrogen (NO₃‐N) in soil is described. Extracting solution [0.025M Al₂(SO₄)₃] and field‐moist soil are measured volumetrically, with NO₃‐N concentration measured by nitrate‐sensitive colorometric test strips or nitrate‐selective electrode. Across a range of soil texture, moisture content, and NO₃‐N concentration, the procedure was well correlated with conventional laboratory analysis of 2N KC1 soil extracts (r² = 0.94). This quick test procedure is proposed as an on‐farm monitoring technique to improve N management.     

Influence of organic compounds on nitrogen‐fertilizer solubilization

Abstract

In the attempt to find new products which release nutrients in gradual forms, the behavior of two commercial fertilizers was studied, Nitrophoska® (N) and urea (U), covered with two organic materials, humic acid (HA) and alginic acid (AA). The release of nitrogen from the fertilizers was determined by electroultrafiltration (EUF). These applied materials on the fertilizer surface resulted in a slowing of the release of nitrogen, although strictly speaking, these compounds do not function as coated fertilizers. Their effectiveness depends on the fertilizer, for with Nitrophoska®, the addition of alginic acid was more effective, while for urea, the addition of humic acid slowed the release of nitrogen.     

Do carbon farming practices build bioavailable nitrogen pools?

Agricultural soils contain large amounts of nitrogen (N), but only a small fraction is readily available to plants. Despite several methods developed to estimate the bioavailability of N, there is no consensus on which extraction methods to use, and which N pools are critically important. In this study, we measured six soil N pools from 20 farms, which were part of a multi-year soil carbon sequestration on-farm experiment (Carbon action, 2019–2023). The aim was to quantify the N pools and to evaluate if farming practices that aim to build soil carbon pools, also build bioavailable N pools. We also aimed to test if the smaller and rapidly changing N pools could serve as an indicator for the slower change in soil organic matter. The measured N pools decreased in size, when moving from total N (7700 ± 1500 kg/ha) to slowly cycling (Illinois Soil Nitrogen Test ISNT-N: 1063 ± 220 kg/ha, autoclave citrate-extracted ACE protein N: 633 ± 440 kg/ha), water-soluble organic N (50 ± 17 kg/ha), potentially mineralizable N (33 ± 13 kg/ha) and finally readily plant available inorganic pools (nitrate and ammonium, total: 14 ± 8 kg/ha). In total, the measured pools covered only 18%–44% of total N, indicating a large unidentified N pool, which is either tightly bound to soil mineral fraction and not easily extractable or is bound to undecomposed plant residues and not hydrolysed by the methods. Of the large N pools (ISNT-N, ACE protein and unidentified residual N), clay, carbon (C) and C:Clay ratios explained most of the variability (R² = .90–.93), leaving a minor part of the variation to the management effect. A pairwise comparison of carbon farming and control plots concluded that farming practices had a small (3%–5%) but statistically significant (p < .05) effect on soil total N and ISNT-N pools, and a moderate and significant effect (18%, p < .01) on potentially mineralizable N. The large variation in protein N, water-soluble organic N and inorganic N reduced statistical significance, although individual C sequestration practices had large effects (−30% to +50%). In conclusion, carbon sequestration practices can build both slowly cycling N pools (ISNT) and increase the mineralisation rate of these pools to release plant available forms, resulting in an additional benefit to agriculture through reduced fertilizer application needs.     

Influence of Azospirillum inoculation and nitrogen supply on grain yield,and carbon‐ and nitrogen‐assimilating enzymes in maize

Nitrogen (N) supply increased yield, leaf % N at 10 days after silking (DAS) and at harvesting, the contents of ribulose‐1,5‐bisphosphate carboxylase (RUBISCO) and soluble protein, and the activities of phosphoenolpyruvate carboxylase (PEPC), and ferredoxin‐glutamate synthase (Fd‐GOGAT), but not of glutamine synthetase (GS) for six tropical maize (Zea mays L) cultivars. Compared to plants fertilized with 10 kg N/ha, plants inoculated with a mixture of Azospirillum sp. (strains Sp 82, Sp 242, and Sp Eng‐501) had increased grain % protein, and leaf % N at 10 DAS and at harvest, but not grain yield. Compared to plants fertilized with either 60 or 180 kg N/ha, Azospirillum‐inoculated plants yielded significantly less, and except for GS activity, which was not influenced by N supply, had lower values for leaf % N at 10 DAS and at harvest, for contents of soluble protein and RUBISCO, and for the activities of PEPC and Fd‐GOGAT. Yield was positively correlated to leaf % N both at 10 DAS and at harvest, to the contents of soluble protein and RUBISCO, and to the activities of PEPC and Fd‐GOGAT, but not of GS, when RUBISCO contents and enzyme activities were calculated per g fresh weight/min. However, when enzyme contents and enzyme activities were expressed per mg soluble protein/min, yield was correlated positively to RUBISCO and PEPC, but negatively to GS. These results give support to the hypothesis that RUBISCO, Fd‐GOGAT, and PEPC may be used as biochemical markers for the development of genotypes with enhanced photosynthetic capacity and yield potential.     

Carbon and nitrogen turnover in response to warming and nitrogen addition during early stages of forest litter decomposition—an incubation experiment

Purpose

Little is known about the interactive effects of temperature, nitrogen (N) supply, litter quality, and decomposition time on the turnover of carbon (C) and N of forest litter. The objective of this study was to investigate the interactive effects of warming, N addition and tree species on the turnover of C and N during the early decomposition stage of litters in a temperate forest.

Materials and methods

A 12-week laboratory incubation experiment was carried out. The leaf litters including two types of broadleaf litters (Quercus mongolica and Tilia amurensis), a needle litter (Pinus koraiensis), and a mixed litter of them were collected from a broad-leaved Korean pine mixed forest ecosystem in northeastern China in September 2009. Nine treatments were conducted using three temperatures (15, 25, and 35 °C) combined with three doses of N addition (equal to 0, 75, and 150 kg?·?ha^?1?a^?1, respectively, as NH₄NO₃).

Results and discussion

After 12 weeks of incubation, the mass loss ranged between 12 and 35 %. The broadleaf litters had greater mass loss and cumulative CO₂–C emission than the needle litter. Temperature and N availability interacted to affect litter mass loss and decomposition rate. The dissolved organic carbon (DOC) and nitrogen (DON) concentrations in litter leachate varied widely with litter types. DOC increased significantly with increased temperature but decreased significantly with increased N availability. DON increased significantly with increased N availability but showed a higher level at the moderate decomposition temperature. The amounts of CO₂ and N₂O emission were significantly higher at 25 °C than those at 15 and 35 °C, and were significantly increased by the N addition.

Conclusions

The present study indicated relatively intricate temperature and N addition effects on C and N cycling during early stages of litter decomposition, implying that future increases in temperature and N deposition will directly affect C and N cycling in broad-leaved Korean pine mixed forest ecosystem, and may indirectly influence the ecosystem composition, productivity, and functioning in NE China. It is, therefore, important to understand the interactive effects of biotic and abiotic factors on litter decomposition in field conditions in order to assess and predict future ecosystem responses to environmental changes in NE China.