Temperature functions of the rate coefficients of net N mineralization in sandy arable soils. Part I. Derivation from laboratory incubations

This study aimed to experimentally determine adequate temperature functions for the rate coefficients of net N mineralization in sandy arable soils from NW Germany. Long‐term laboratory incubations were carried out in seven sandy arable soils at 3°C, 10°C, 19°C, 28°C, and 35°C in order to derive the rate coefficients of a simultaneous two‐pool first‐order kinetic equation. Thereby we differentiated between a small, fast mineralizable N pool, comprising mainly fresh residues, and a larger, slowly mineralizable N pool of old, humified organic matter. The rate coefficients were plotted against temperature, and fits of several different functions were tested: Arrhenius, Q₁₀, and multiple non‐mechanistic equations. The two derived rate coefficients showed very different temperature functions. Especially in critical temperature ranges (<5/10°C, >30/35°C) common Q₁₀ functions failed to fit well, and, only below 10°C, the Arrhenius functions were in agreement with mean measured rate coefficients. Over the studied temperature range, only relatively complex, multiple equations could adequately account for the observed patterns. In addition, temperature functions that have been derived earlier from loess soils from NW Germany were found not to be transferable to the sandy arable soils studied. Thus, the results strongly question the use of the same Arrhenius or Q₁₀ function or the same rate modifying factor for different N pools as well as for different soils as is generally done in models. Evaluations with field measurements of net N mineralization in part II of the paper (Heumann and Böttcher, 2004) will show which functions perform best in the field.     

N mineralization parameters of sandy arable soils

The objective of this study was to experimentally investigate net N mineralization in sandy arable soils and to derive adequate N mineralization parameters for simulation purposes. Long‐term incubations at 35 °C were done for at least 200 days with 147 sandy arable soils from Northwest Germany. To cumulative net N mineralization curves the simultaneous two‐pool first‐order kinetic equation was fitted in order to differentiate between N mineralization from an easily decomposable, fresh organic matter pool (N_fast) and from a slowly decomposable pool (N_slow) of more humified OM. North German loess soils served as a reference, since available model parameters were mainly derived from those soils. Although curve patterns in sandy soils often somewhat deviated from typical double‐exponential patterns, the mineralization equation generally could be fitted. Two pools were clearly revealed, but a transfer of the standard parameters was found to be not appropriate — except maybe for the pool size of the fast decomposable N pool. The mean k_fast at 35 °C (0.1263 d^—1) is about 46% higher than the known ’︁standard’ loess value, indicating better conditions for decomposition of fresh residues at this temperature. The mean k_slow at 35 °C (0.0023 d^—1), which is 60% lower than reported earlier from loess soils, and much lower mineralization rates of the slowly decomposable N pool give reason to the presence of generally more resistant organic material in these sandy soils. The relation between N_slow and total N was found to be not close enough to derive the pool size of slowly decomposable N just from total N as done for loess soils. Reducing the variability is necessary, promising approaches exist. The eight reference loess soils revealed — on an average — the known N mineralization parameters.     

Pedotransfer functions for the pool size of slowly mineralizable organic N in sandy arable soils

The major aim of this study was to evaluate how the pool size of slowly mineralizable, ‘old’ soil organic N can be derived from more easily accessible soil and site information via pedotransfer functions (PTF). Besides modeling, this pool size might be of great importance for the identification of soils with high mineralization potential in drinking‐water catchments. From long‐term laboratory incubations (ca. 200 days) at 35 °C, the pool sizes of easily mineralizable organic N (N_fast), mainly in fresh residues, and slowly mineralizable, ‘old’ soil organic N (N_slow) as well as their first‐order rate coefficients were obtained. 90 sandy arable soils from NW Germany served to derive PTFs for N_slow that were evaluated using another 20 soils from the same region. Information on former land‐use and soil type was obtained from topographical, historical, and soil maps (partly from 1780). Pool size N_slow very strongly depends on soil type and former land‐use. Mean pool sizes of N_slow were much lower in old arable lowland (105 mg N kg^–1) than upland soils (175 mg N kg^–1) possibly due to lower clay contents. Within lowlands, mean pool sizes in former grassland soils (245 mg N kg^–1) were 2 to 3 times larger than in old arable soils due to accumulation of mineralizable N. In contrast, mean pool sizes of N_slow were lowest in recently cleared, former heath‐ and woodland (31 mg N kg^–1) as a result of the input of hardly decomposable organic matter. Neither N nor C in the light fraction (density < 1.8 g cm^–3) was adequate to derive pool size N_slow in the studied soils (r² < 0.03). Instead, N_slow can be accurately (r² = 0.55 – 0.83) derived from one or two basic soil characteristics (e.g. organic C, total N, C : N, mineral fraction < 20 μm), provided that sites were grouped by former land‐use. Field mineralization from N_slow during winter (independent data set) can be predicted as well on the basis of N_slow‐values calculated from PTFs that were derived after grouping the soils by former land‐use (r² = 0.51***). In contrast, using the PTF without soil grouping strongly reduced the reliability (r² = 0.16).     

Bulk soil C to N ratio as a simple measure of net N mineralization from stabilized soil organic matter in sandy arable soils

Investigations of 23 northwestern German sandy Ap horizons (mean clay content 35 g kg⁻¹), that had higher organic matter (OM) levels than expected for sands, showed that the bulk soil C to N ratio reliably indicated the release of N from stabilized OM. Soils were incubated at 35 °C for 200 days under aerobic conditions. Cumulative N release curves were split into N released from fresh materials (N_fast) and N released from the larger pool of stabilized, older OM (N_slow rates, 0.06-0.77 μg N g⁻¹ soil d⁻¹, or 0.7-49 μg N g⁻¹ OM). Correlating the N_slow rates with total N contents of soils yielded no satisfactory relationships while their relationship with C to N ratios was very close (negative exponential, R²=0.88). Low rates of N release (N_slow) per unit of OM occurred if C to N exceeded 15. This was associated with historical factors like podzolization, calluna heathland, plaggen fertilization or a combination of these.     

Organic matter fractions and N mineralization in vegetable‐cropped sandy soils

Soil organic nitrogen mineralization rates and possible predictors thereof were investigated for vegetable‐growing soils in Belgium. Soil organic matter (SOM) was fractionated into sand (> 53 μm) and silt+clay (< 53 μm) fractions. The latter fraction was further separated into 6%NaOCl‐oxidation labile (6%NaOCl‐ox) and resistant N and C and subsequently into 10%HF‐extractable (mineral bound) and resistant (recalcitrant) N and C. The N mineralization turnover rate (% of soil N/year) correlated with several of the investigated N or C fractions and stepwise linear regression confirmed that the 6%NaOCl‐ox N was the best predictor. However, the small R² (0.42) of the regression model suggests that soil parameters other than the soil fractions isolated here would be required to explain the significant residual variation in N mineralization rate. A next step could be to look for alternative SOM fractionations capable of isolating bioavailable N. However, it would appear that the observed relationships between N fractions and N mineralization may not be causal but indirect. The number of vegetable crops per rotation did not influence N mineralization, but it did influence 6%NaOCl‐ox N, probably as an effect of differences in crop residues returned and organic manure supply. However, the nature of this relation between management, SOM quality and N mineralization is not clear. Explanation of correlations between N mineralization and presumed bioavailable N fractions, like the 6%NaOCl‐ox N, requires further mechanistic elucidation of the N mineralization process.     

Nitrogen fertilizer–induced mineralization of soil organic C and N in six contrasting soils of Bangladesh

A 90‐day laboratory incubation study was carried out using six contrasting subtropical soils (calcareous, peat, saline, noncalcareous, terrace, and acid sulfate) from Bangladesh. A control treatment without nitrogen (N) application was compared with treatments where urea, ammonium sulfate (AS), and ammonium nitrate (AN) were applied at a rate of 100 mg N (kg soil)^–1. To study the effect of N fertilizers on soil carbon (C) turnover, the CO₂‐C flux was determined at nine sampling dates during the incubation, and the total loss of soil carbon (TC) was calculated. Nitrogen turnover was characterized by measuring net nitrogen mineralization (NNM) and net nitrification (NN). Simple and stepwise multiple regressions were calculated between CO₂‐C flux, TC, NNM, and NN on the one hand and selected soil properties (organic C, total N, C : N ratio, CEC, pH, clay and sand content) on the other hand. In general, CO₂‐C fluxes were clearly higher during the first 2 weeks of the incubation compared to the later phases. Soils with high pH and/or indigenous C displayed the highest CO₂‐C flux. However, soils having low C levels (i.e., calcareous and terrace soils) displayed a large relative TC loss (up to 22.3%) and the added N–induced TC loss from these soils reached a maximum of 10.6%. Loss of TC differed depending on the N treatments (urea > AS > AN >> control). Significantly higher NNM was found in the acidic soils (terrace and acid sulfate). On average, NNM after urea application was higher than for AS and AN (80.3 vs. 71.9 and 70.9 N (kg soil)^–1, respectively). However, specific interactions between N‐fertilizer form and soil type have to be taken into consideration. High pH soils displayed larger NN (75.9–98.1 mg N (kg soil)^–1) than low pH soils. Averaged over the six soils, NN after application of urea and AS (83.3 and 82.2 mg N (kg soil)^–1, respectively) was significantly higher than after application of AN (60.6 mg N (kg soil)^–1). Significant relationships were found between total CO₂ flux and certain soil properties (organic C, total N, CEC, clay and sand content). The most important soil property for NNM as well as NN was soil pH, showing a correlation coefficient of –0.33** and 0.45***, respectively. The results indicate that application of urea to acidic soils and AS to high‐pH soils could be an effective measure to improve the availability of added N for crop uptake.     

Evaluation of two N cycle models for the prediction of N mineralization from grassland soils in the UK

Abstract. The ability of two nitrogen cycle models, of contrasting complexity, to predict N mineralization from a range of grassland soils in the UK, was evaluated. These were NCYCLE, a simple mass balance model of the N cycle in UK grasslands, and CENTURY, a more complex model simulating long-term C, N, P & S dynamics in grassland ecosystems. The models were tested using field measurements of net N mineralization from a range of grassland soils (differing in soil type, history & management practice), obtained over a 2 year period using a soil core incubation technique. This method was considered to measure the total net release of mineral N from the soil organic matter over a specified time, including N which may have been recycled several times. NCYCLE consistently under-estimated mineralization rates at all sites. By contrast, there was some correlation between CENTURY predictions of net N mineralization and field measurements. This may have reflected the different abilities of the two models to simulate N recycling. Neither model, however, was able to predict adequately the effect of cultivation and reseeding on net N mineralization.     

Nitrate leaching loss following application of organic manures to sandy soils in arable cropping.

Abstract. Experiments were set up at two sites to measure nitrogen (N) leaching loss from applications of separated pig/cattle slurry and cattle farmyard manure(FYM), during winters 1990/91–1993/94 (site A) and from broiler litter and FYM, during winters 1990/91–1992/93 (site B). The manures were applied at a target rate of 200 kg ha^-1 total N during the autumn and winter to overwinter fallow or top dressed onto winter rye. The total N in leachate was calculated from leachate N concentrations, in samples collected using ceramic cups buried at 90 cm, and an estimate of drainage volume. Nitrogen losses were greatest following manure applications in September, October and November but losses following applications in December or January were not significantly elevated above those from untreated controls. Losses were consistently lower from FYM than from broiler litter or separated slurry. The presence of a cover crop (winter rye) significantly reduced overall N leaching compared with the fallow, but only reduced the manure N leaching losses at one site during one winter when a high proportion of drainage occurred late. The incorporation of a nitrification inhibitor (DCD) with manures applied in October did not significantly reduce the manure N leaching.     

Nitrogen mineralization from mature bio‐waste compost in vineyard soils. III Simulation of soil mineral‐nitrogen dynamics

A model for nitrogen (N) dynamics in compost‐amended vineyard soils was tested for its predictive power. A soil–mineral N data set from a 3‐year field study on four different vineyard sites was used for model evaluation. The soils were treated with mature bio‐waste compost (30 and 50 Mg ha^–1 fresh matter, respectively). The model calculated soil mineral‐N contents at all sites with an overall mean bias error of –2.2 kg N ha^–1 for layers of 0.1 m thickness and an overall mean absolute error of 7.4 kg N ha^–1 layer^–1. Modeling efficiencies for the simulations of the respective treatments ranged from –0.05 to 0.41, and Willmott's Index of Agreement showed values of between 0.41 and 0.81. Acceptable model predictions as defined by the observed variability of mineral‐N contents in the respective soils ranged from 40% to 72%. A strong increase in soil mineral‐N concentration following the compost application at all sites could not be reproduced with the model, thereby reducing the prediction accuracy significantly. The model performance confirms that previously derived N‐mineralization parameters are suitable to describe the N release from soil‐applied mature bio‐waste compost under the environmental conditions of vineyards in Germany.     

The effect of a perennial cover crop on net soil N mineralization and microbial biomass carbon in coconut plantations in the humid tropics

The decrease in soil fertility in agroecosystems due to continuous harvesting, loss of fine soil and oxidation of soil organic carbon (SOC) is well known. This study evaluates for a humid tropical climate in South Andaman Island, India, the impact of a 15-yr old Pueraria cover crop (CC) under a coconut plantation, with and without phosphorus (P) application, on the soil N mineralization rate (NMR), the mineral N pool (NH₄⁺-N and NO₃⁻-N), microbial biomass carbon (MB-C) and SM under four treatments, (1) no cover crop (NCC), (2) no cover crop + phosphorus (NCC+P), (3) CC, and (4) cover crop + phosphorus (CC+P) during three seasons, wet (May–October), post-wet (November–January) and dry (February–April). The NCC treatment served as a control. In addition, an ex-situ experiment was conducted to verify the effect of P application on NMR and MB-C under 100% field capacity (FC), 50 and 25% FC representing the different seasons. The NMR, mineral N pool and MB-C increased by 37, 46 and 41%, respectively under the CC compared to the control. SOC and fine soil particles were also greater under the CC by 41 and 461%, respectively, compared to the control. The application of P to the CC increased soil N mineralization, the mineral N pool and MB-C by 33, 16 and 14%, respectively. The amount of mineralized N was greater under the CC and CC+P treatments by 39 and 73%, respectively than the control. The ex-situ experiment showed that the P application increased NMR and MB-C, but the increases were highest in the 50% FC and lowest in 100% FC. It is proposed for the humid tropics that a CC could be used for enhancing SOC and increasing soil N mineralization under coconut plantations and other similar agroecosystems.     

N mineralization process of the surface soils under shifting cultivation in northern Thailand

N mineralization process (ammonification plus nitrification) in the surface 0-5 cm soil layers under shifting cultivation in northern Thailand was studied. Labile pool of organic matter extracted with a K₂S0₄ solution at 1l0°C in an autoclave (fraction A) or by shaking at room temperature (fraction B) was used as factor to evaluate the N mineralization process which was examined in an incubation experiment. In the soils, in which the N mineralization pattern was fitted to a first order kinetics model, the content of (organic + NH₄ ⁺)-N in fraction B determined the initial rate of N mineralization. The soils, which showed a short lag time of less than 7 d both in the N mineralization and nitrification processes, had a high ratio of organic C to (organic + NH₄ ⁺)-N in fraction B, exceeding the value of 7. The soils, which showed a long lag time of more than 7 d only in the nitrification process, had a low pH(KCI) (less than 4.2). Thus, the rate of N mineralization was affected by the labile pool in fraction B or soil pH. On the other hand, there was a correlation between the N ₀ + N _max (inorganic N at 0 d + maximum amount of mineralizable N) value and the labile pool in the fraction A, suggesting that the N ₀ + N _max value depended on the contents of the labile pool.     

锡林河流域温带草原土壤的净氮矿化研究

氮素矿化是决定土壤供氮能力的重要过程,也是目前国内外研究的热点。该文采用树脂芯方法测定了内蒙古锡林河流域不同温带草原土壤在雨季期间的净氮矿化率,对树脂芯方法在温带草原的应用效果进行了评价。结果表明,实验期间贝加尔针茅草原、羊草草原和大针茅草原土壤的平均日净氮矿化率分别为0.035、0.120和0.125 kg/(hm²·d);树脂芯方法对草原土壤氮转化过程干扰较小,是自然条件下研究温带草原土壤净氮矿化的有效手段。     

Effect of temperature on the mineralization of C and N of fresh and mature compost in sandy material

Information about the mineralization rate of compost at various temperatures is a precondition to optimize mineral N fertilization and to minimize N losses in compost‐amended soils. Objectives were to quantify the influence of the temperature on the mineralization rate and leaching of dissolved organic carbon (DOC) and nitrogen (DON), NO₃^—, and NH₄⁺ from a fresh (C : N = 15.4) and a mature (C : N = 9.2) organic household waste compost. Compost samples were mixed with quartz sand to ensure aerobic conditions, incubated at 5, 10, 15, 20, and 25°C and irrigated weekly for 112 days. For the fresh compost, cumulative CO₂ evolution after 112 days ranged from 36% of the initial C content at 5°C to 54% at 25°C. The CO₂ evolution was only small in the experiments with mature compost (1 to 6% of the initial C content). The data were described satisfactorily by a combined first‐order (fresh compost) or a first‐order kinetic model (mature compost). For the fresh compost, cumulative DOC production was negatively related to the temperature, probably due to leaching of some of the partly metabolized easily degradable fractions at lower temperatures. The production ratios of DOC : CO₂‐C decreased with increasing temperature from 0.094 at 5°C to 0.038 at 25°C for the fresh and from 1.55 at 5°C to 0.26 at 25°C for the mature compost. In the experiments with fresh compost, net release of NO₃^— occurred after a time lag which depended on the temperature. Cumulative net release of NO₃^— after 112 days ranged from 1.8% of the initial N content at 5°C to 14.3% at 25°C. Approximately 10% of the initial N content of the mature compost was released as NO₃^— after 14 days at all temperatures. The DOC : DON ratios in the experiments using fresh compost ranged from 11.5 to 15.7 and no temperature dependency was observed. For the mature compost, DOC : DON ratios were slightly smaller (7.4 to 8.9). The DON : (NH₄⁺ + NO₃^—) ratio decreased with increasing temperature from 0.91 at 5°C to 0.19 at 25°C for the fresh compost and from 0.21 at 5°C to 0.12 at 25°C for the mature compost. The results of the dynamics of C and N mineralization of fresh and mature compost can be used to assess the appropriate application (timing and amount) of compost to soils.     

Nitrogen mineralization from mature bio‐waste compost in vineyard soils II. Test of N‐mineralization parameters in a long‐term in situ incubation experiment

A field incubation experiment was carried out to test the applicability of N‐mineralization parameters for mature bio‐waste compost for use in a simulation model. The parameters were previously obtained from a laboratory experiment. Micro‐lysimeters were used for incubation, containing four different vineyard soils that were treated with three different compost‐application rates (0, 30, and 50 Mg compost ha^–1). Between 2.0% and 45.2% of total bio‐waste compost N was mineralized and leached from the micro‐lysimeters during the two‐year investigation period. The application of a simulation model for soil N dynamics revealed two major drawbacks of the model: (1) in most of the soils, extraordinary high mineralization rates were observed within a few weeks after compost amendment, which could not be explained by the model, and (2) the average compost‐N‐mineralization rates were estimated as being close to the observed rates (–6%), but distinct deviations in some cases (–46% to +29%) led to considerable miscalculations in long‐term simulations. Excluding the effect of these two processes from the data set, the remaining variance could be well explained by the model for all soils treated with compost (modeling efficiency ≥0.98). Based on the average performance, the mineralization parameters for mature bio‐waste compost are considered to be applicable for use in any simulation model based on the double‐exponential approach for calculating fertilizer recommendations, whereas the functions calculating the impact of environmental factors on N mineralization in the model need to be revised. The initial mineralization flush observed in most of the compost treatments was attributed to a priming effect. The experiment showed that such a priming effect can cause exceptionally high rates of N mineralization from mature bio‐waste compost in a viticultural environment, which exceed the potential mineralization rates known for bio‐waste compost applied to arable soils in Germany.     

Impact of Acacia auriculiformis on the chemical fertility of sandy soils on the Batéké plateau,D.R. Congo

A 17‐year chronosequence of Acacia auriculiformis fallows on Arenosols of the Batéké Plateau (D.R. Congo) was surveyed and compared with virgin savannah soils to assess chemical soil fertility changes induced by these N‐fixing trees. Significant increases in organic carbon content, total nitrogen content, cation exchange capacity and sum of base cations were found after relatively short fallow periods of only 4 years and did not only affect the forest floor, but extended to at least 50 cm depth. The Acacia act as a major source of organic matter (OM), hence increasing organic carbon and nitrogen content and decreasing the C/N ratio. The increased OM content suggests that humification processes are the main cause of the significant decrease in pH. Total exchangeable cations initially increased slowly but doubled (topsoil 0–25 cm) and tripled (subsoil 25–50 cm) after 10 years. The point of zero net proton charge was systematically lower than soil pH and decreased with increasing OM content, thereby increasing the cation exchange capacity, although concurrent acidification retarded a significant beneficial impact at field pH on Acacia fallows of 10 years and older. Although the chemical soil fertility improves steadily with time, after 8 years of Acacia fallow the absolute amounts of available nutrients are still small and slash and burn practices are required to liberate the nutrients stored in the remaining biomass and litter before each new cropping period.     

Use of long-term monitoring data to derive a relationship between nitrogen surplus and nitrate leaching for grassland and arable land on well-drained sandy soils in the Netherlands

The decrease in nitrogen (N) use in agriculture led to improvement of upper groundwater quality in the Sand region of the Netherlands in the 1991–2009 period. However, still half of the farms exceeded the European nitrate standard for groundwater of 50 mg/l in the 2008–2011 period. To assure that farms will comply with the quality standard, an empirical model is used to derive environmentally sound N use standards for sandy soils for different crops and soil drainage conditions. Key parameters in this model are the nitrate-N leaching fractions (NLFs) for arable land and grassland on deep, well-drained sandy soils. NLFs quantify the fraction of the N surplus on the soil balance that leaches from the root zone to groundwater and this fraction represents N available for leaching and denitrification. The aim of this study was to develop a method for calculating these NLFs by using data from a random sample of commercial arable farms and dairy farms that were monitored in the 1991–2009 period. Only mean data per farm were available, which blocked a direct derivation of NLFs for unique combinations of crop type, soil type and natural soil drainage conditions. Results showed that N surplus leached almost completely from the root zone of arable land on the most vulnerable soils, that is, deep, well-drained sandy soils (95% confidence interval of NLF 0.80–0.99), while for grassland only half of the N surplus leached from the root zone of grassland (0.39–0.49). The NLF for grassland decreased with 0.015 units/year, which is postulated to be due to a decreased grazing and increased year-round housing of dairy cows. NLFs are positively correlated with precipitation surplus (0.05 units/100 mm for dairy farms and 0.10 units/100 mm for arable farms). Therefore, an increase in precipitation due to climate change may lead to an increase in leaching of nitrate.     

Importance of organic nitrogen fractions in sandy soils,obtained by electro-ultrafiltration or CaCl2 extraction,for nitrogen mineralization and nitrogen uptake of rape

Summary Sandy soils have low reserves of mineral N in spring. Therefore organic-bound N is the most important pool available for crops. The objective of the present investigation was to study the importance of the organic-bound N extracted by electro-ultrafiltration and by a CaCl₂ solution for the supply of N to rape and for N mineralization. Mitscherlich-pot experiments carried out with 12 different sandy soils (Germany) showed a highly significant correlation between the organic N extracted (two fractions) and the N uptake by the rape (electroultrafiltration extract: r=0.76^***; CaCl₂ extract: r=0.76^***). Organic N extracted by both methods before the application of N fertilizer was also significantly correlated with N mineralization (electro-ultrafiltration extract: r=0.75^***; CaCl₂ extract: r=0.79^***). N uptake by the rape and the mineralization of organic N increased with soil pH and decreased with an increasing C:N ratio and an increasing proportion of sand in the soils. Ninety-eight percent of the variation in N uptake by the rape was determined by the differences in net mineralization of organic N. This show that in sandy soils with low mineral N reserves (NO _inf3 ^sup- -N, NH ₄ ⁺ -N) the organic soil N extracted by electro-ultrafiltration or CaCl₂ solutions indicates the variance in plant-available N. Total soil N was not related to the N uptake by plants nor to N mineralization.     

Application of the DNDC model to predict N2O emissions from sandy arable soils with differing fertilization in a long‐term experiment

Modeling crop growth and soil N dynamics is difficult due to the complex nature of soil–plant systems. In several studies, the DNDC model has been claimed to be well‐suited for this purpose whereas in other studies applications of the model were less successful. Objectives of this study were to test a calibration and validation scheme for DNDC‐model applications to describe a field experiment with spring wheat on a sandy soil near Darmstadt (SW Germany) using different fertilizer types (either application of mineral fertilizer and straw—MSI; or application of farmyard manure—FYM) and rates (low—MSI_L, FYM_L; and medium—MSI_M, FYM_M). The model test is based on a model parameterization to best describe the case MSI_L and applies this parameterization for a retrospective simulation of the other cases (MSI_M, FYM_L, FYM_M) including crop growth and N₂O emissions. Soil water contents were not accurately simulated using either the DNDC default values for a loamy sand or for the next finer texture class or using results from the pedotransfer function provided by ROSETTA. After successful calibration of the soil water flow model using a soil texture class that led to the best fit of the measured water content data, grain yield of spring wheat and cumulative N₂O emission were slightly underestimated by DNDC and were between 91% and 86% of the measured data. A subsequent calibration of the yields and cumulative N₂O emissions from soils of the MSI_L treatment gave a good prediction of crop growth and N₂O emissions in the MSI_M treatment, but a marked underestimation of yields of the FYM treatments. Cumulative N₂O emissions were predicted well for all MSI and FYM treatments, but seasonal dynamics were not. Overall, our results indicated that for the sandy soil in Germany, site‐specific calibration was essentially required for the soil hydrology and that a calibration was useful for a subsequent prediction where greater amounts of the same fertilizer were used, but not useful for a prediction with a different fertilizer type.     

Denitrification and C,N mineralization as function of temperature and moisture potential in organic and mineral horizons of an acid spruce forest soil

The influence of temperature (T) and water potential (ψ) on the denitrification potential, C and N mineralization and nitrification were studied in organic and mineral horizons of an acid spruce forest soil. The amount of N₂O emitted from organic soil was 10 times larger than from the mineral one. The maximum of N₂O emission was in both soils at the highest water potential 0 MPa and at 20°C. CO₂ production in the organic soil was 2 times higher than in mineral soil. Net ammonification in organic soil was negative for most of the T‒ψ variations, while in mineral soil it was positive. Net nitrification in organic soil was negative only at the maximum water potential and temperature (0 MPa, 28°C). The highest rate was between 0 and −0.3 MPa and between 20 and 28°C. In mineral soil NO₃⁻ accumulated at all T‒ψ variations with a maximum at 20^oC and −0.3 MPa. We concluded that in organic soil the immobilization of NH₄⁺ is the dominant process in the N‒cycling. Nevertheless, decreasing of total N mineralized at 0 MPa and 20—28^oC can be explained by denitrification.     

Microbial biomass,N mineralization,and the activities of various enzymes in relation to nitrate leaching and root distribution in a slurry-amended grassland

High rates of cattle slurry application induce NO _inf3 ^sup- leaching from grassland soils. Therefore, field and lysimeter trials were conducted at Gumpenstein (Austria) to determine the residual effect of various rates of cattle slurry on microbial biomass, N mineralization, activities of soil enzymes, root densities, and N leaching in a grassland soil profile (Orthic Luvisol, sandy silt, pH 6.6). The cattle slurry applications corresponded to rates of 0, 96, 240, and 480 kg N ha^-1. N leaching was estimated in the lysimeter trial from 1981 to 1991. At a depth of 0.50 m, N leaching was elevated in the plot with the highest slurry application. In October 1991, deeper soil layers (0–10, 10–20, 20–30, 30–40, and 40–50 cm) from control and slurry-amended plots (480 kg N ha^-1) were investigated. Soil biological properties decreased with soil depth. N mineralization, nitrification, and enzymes involved in N cycling (protease, deaminase, and urease) were enhanced significantly (P<0.05) at all soil depths of the slurry-amended grassland. High rates of cattle slurry application reduced the weight of root dry matter and changed the root distribution in the different soil layers. In the slurry-amended plots the roots were mainly located in the topsoil (0–10 cm). As a result of this study, low root densities and high N mineralization rates are held to be the main reasons for NO _inf3 ^sup- leaching after heavy slurry applications on grassland.