C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at −10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg⁻¹ were horizontially sliced and ¹⁵N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20 °C and the evolution of CO₂-C determined continuously for 177 d. Inorganic and microbial biomass N and ¹⁵N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO₃-N present in soil before faeces was applied, net nitrification became negative indicating that NO₃-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and ¹⁵N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces ¹⁵N was recovered in the microbial biomass in the amended soils. CO₂-C evolution increased with clay content in amended and unamended soils. CO₂-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the soil water content.     

Winter cover cropping influence on nitrogen mineralization,presidedress soil nitrate test,and corn yields

The mineralization and availability of cover crop N to the succeeding crop are critical components in the management of soil N to reduce N leaching. The effects of several leguminous and non-leguminous cover crops on soil N availability, N mineralization potential, and corn (Zea mays L.) yield were examined. The cover crops had variable effects on soil N availability and corn yield and N uptake. Because of the rapid mineralization of the cover crops following incorporation, the inorganic N levels in the soil sampled in mid-May 1992 (4 weeks after incorporation of cover crops), rather than the potentially mineralizable N, rate constants, initial potential mineralization rate, or cumulative N mineralized over 14 weeks, correlated well with N concentrations, C:N ratios, or the N added in the cover crops. However, the inclusion of potentially mineralizable N with inorganic N in a multiple regression improved the variability in the corn yield and the N uptake accounted for. Since extensive mineralization had occurred before the 21 May sampling, the potentially mineralizable N was affected more by the soil organic N and C than by the N concentrations of the cover crops. The presidedress NO₃ ^--N test levels were well predicted by the inorganic and potentially mineralizable N (R ²=0.89, P<0.01), although the test levels were better in predicting corn yield and N uptake. If the available soil N test needs to be made earlier than recommended by the presidedress NO₃ ^--N test, both inorganic and potentially mineralizable N are needed to better predict the corn yield and N uptake in the soils.     

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).     

Microbial and soil parameters in relation to N mineralization in soils of diverse genesis under differing management systems

 Oregon soils from various management and genetic histories were used in a greenhouse study to determine the relationships between soil chemical and biological parameters and the uptake of soil mineralized nitrogen (N) by ryegrass (Lolium perenne L.). The soils were tested for asparaginase, amidase, urease, β-glucosidase, and dipeptidase activities and fluorescein diacetate hydrolysis. Microbial biomass carbon (C) and N as well as metabolic diversity using Biolog GN plates were measured, as were total soil N and C, pH, and absorbance of soil extracts at 270 nm and 210 nm. Potentially mineralizable N (N₀) and the mineralization rate constant (k) were calculated using a first order nonlinear regression model and these coefficients were used to calculate the initial potential rate of N mineralization (N₀ k). Except for Biolog GN plates, the other parameters were highly correlated to mineralized N uptake and each other. A model using total soil N and β-glucosidase as parameters provided the best predictor of mineralized N uptake by ryegrass (R ² =0.83). Chemical and biological parameters of soils with the same history of formation but under different management systems differed significantly from each other in most cases. The calculated values of the initial potential rate of mineralization in some cases revealed management differences within the same soil types. The results showed that management of soils is readily reflected in certain soil chemical and biological indicators and that some biological tests may be useful in predicting N mineralization in soils. Received: 31 January 1997     

Temperature functions of the rate coefficients of net N mineralization in sandy arable soils. Part II. Evaluation via field mineralization measurements

The aim of this study was to evaluate experimentally derived temperature functions for the rate coefficients of net N mineralization in sandy arable soils from NW Germany via field measurements. In part I of this paper (Heumann and Böttcher, 2004), different temperature functions for the rate coefficients of a two‐pool first‐order kinetic equation were derived by long‐term laboratory incubations at 3°C to 35°C. In this paper, field net N mineralization during winter of 25 plots was measured in undisturbed soil columns with a diameter of 20 cm to the depth of the A_p horizon. Mean simulated net N mineralization with the most adequate multiple functions corresponded also best with the mean of the measured values despite of an overestimation of about 10%. Distinctly larger deviations under use of other temperature functions (Arrhenius, Q₁₀) were directly related to their deviations from mean, experimentally derived rate coefficients. Simulated net N mineralization in the soil columns was significantly correlated with measured values, regardless of the temperature functions. Yet the goodness of fit was generally relatively low due to the spatial variability of measured net N mineralization within replicate soil columns, although the mean CV (38%) was by far not extraordinary. The pool of slowly mineralizable N contributed considerably to net N mineralization during four to five winter months, on an average 10.0 kg N ha^–1, about one third of total simulated N mineralization. Sometimes, it contributed even 21.3 kg N ha^–1, which is almost sufficient to reach the EU drinking‐water limit for nitrate in these soils. Simulations with widely used functions that were once derived from loess soils overestimated mineralization from pool N_slow in the studied sandy arable soils by a factor of two.     

Arginine ammonification and L‐glutaminase assays as rapid indices of corn nitrogen availability

Increasing use of N fertilizer for crop production necessitates more rapid estimates on N provided by the soil in order to prevent under‐ or overfertilization and their adverse effect on plant nutrition and environmental quality. A study was conducted to investigate the responses of arginine ammonification (AA), L‐glutaminase activity (LG), soil N–mineralization indices, corn (Zea mays L.) crop–yield estimation, and corn N uptake to application of organic amendments. The relationships between corn N uptake and the microbial and enzymatic processes which are basically related to N mineralization in soil were also studied. The soil samples were collected from 0–15 cm depth of a calcareous soil that was annually treated with 0, 25, or 100 Mg ha^–1 (dry‐weight basis) of sewage sludge and cow manure for 7 consecutive years. Soil total N (TN), potentially mineralizable N (N₀), and initial potential rates of N mineralization (kN₀) were significantly greater in sewage sludge–treated than in cow manure–treated soils. However, the amendment type did not influence soil organic C (SOC), AA, LG, and anaerobic index of N mineralization (N_ana). The application rates proportionally increased N‐availability indices in soil. Corn N concentration and uptake were correlated with indices of mineralizable N. A multiple stepwise model using AA and N_ana as parameters provided the best predictor of corn N concentration (R = 0.86, p < 0.001). Another model using only LG provided the best predictor of corn N uptake (R = 0.78, p < 0.001). This results showed that sewage‐sludge and cow‐manure application is readily reflected in certain soil biological properties and that the biological tests may be useful in predicting N mineralization and availability in soil.     

围海造田长期耕种稻田和旱地土壤氮矿化速率及供氮潜力比较

围海造田是沿海地区拓展土地面积的主要途径。土壤氮矿化参数是揭示围海造田土壤肥力演变和土壤氮供应的重要指标,但是我国沿海造田土壤的相关研究少有报道。本研究以杭州湾南岸海积平原上慈溪市1000年和520年筑塘造田区为对象,选择4个代表性采样点,每个点从低洼稻田采集1个表层混合水稻土,在其相邻高地采集1个表层混合旱地土壤,共8个样品。采用间隙淋洗法研究了土壤样品氮矿化动力学特征。结果如下: 119 d培养试验证实水稻土和旱地土壤有机氮矿化动力学符合一级反应动力学方程Nt=N0(1-e-kt); 水稻土有机氮矿化势（N0）为82.7～161.9 mg/kg（平均114 mg/kg）,占有机氮的7.3%,旱地土壤N0为63.9～104.4 mg/kg（平均83.4 mg/kg）,占有机氮的7.3%; 水稻土有机氮矿化速率（k）为0.033～0.114/d（平均0.064/d）,旱地土壤k为0.007～0.023/d（平均0.020/d）。土壤综合供氮指标（N0k）,水稻土为3.8418.46 mg/(kgd)［平均8.0 mg/(kgd)］,旱地土壤为0.54～2.66 mg/(kgd)［平均1.6 mg/(kgd)］。水稻土总氮含量为1.4～2.0 g/kg （平均1.6 g/kg）,旱地为0.87～2.0 g/kg（平均1.3 g/kg）。可见,水稻土氮库、供氮潜力和速率均大于相邻旱地土壤。因此,从土壤氮肥力来讲,相对于旱地,围海形成的水稻田更具有可持续利用性。     

Prediction of potentially mineralizable N from amidohydrolase activities in a manure-applied, corn residue-amended soil

Prediction of potentially mineralizable N as an important N pool from soil amidohydrolases was investigated. Composite soil samples were collected from plots of a field experiment in which 0, 50 and 100 Mg cow manure ha⁻¹ year⁻¹ had been applied for five consecutive years. The soils were treated with corn shoots or roots or remained untreated in a factorial combination with the manure treatments, with three replications. The mineralized inorganic N was measured periodically in 20-week incubations and potentially mineralizable N (N₀) was calculated based on a first-order kinetic model. Urease, l-glutaminase and l-asparaginase activities were measured before and after incubation. The values of N₀ ranged from 208.6 in the controls to 388.4 in soils that had received 50 Mg ha⁻¹ year⁻¹ of cow manure and were amended with corn shoots. Corn residue amendment in the manure treated soils, increased the values of N₀ or changed the N mineralization kinetic pattern from a first-order to a zero-order model. According to a relative sensitivity index, l-asparaginase was the most sensitive enzyme to the treatments. Multiple regression analysis revealed that 92% of N₀ variations can be described by the activities of urease and l-asparaginase and therefore the soil amidohydrolase activities have the potential to evaluate potentially mineralizable N.     

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.     

The effect of maize straw placement on mineralization of C and N in soil particle size fractions

Fundamental knowledge about the complex processes during the decomposition, mineralization and transfer of residue organic matter in soils is essential to assess risks of changes in agricultural practices. In a double tracer (¹³C, ¹⁵N) experiment the effect of maize straw on the mineralization dynamics and on the distribution of maize-derived organic matter within particle size fractions was investigated. Maize straw (a C₄ plant) labelled with ¹⁵N was added to soils (13.2 g dry matter kg^–1 soil) which previously had grown only C₃ plants, establishing two treatments: (i) soil mixed with maize straw (mixed), and (ii) soil with maize straw applied on the surface (surface). Samples were incubated in the laboratory at 14°C for 365 days. The size fractions (> 200 μm, 200–63 μm, 63–2 μm, 2–0.1 μm and < 0.1 μm), obtained after low-energy sonication (0.2 kJ g^–1), were separated by a combination of wet-sieving and centrifuging. The mineralization of maize C was similar in the two treatments after one year. However, decomposition of maize particulate organic matter (predominantly in the fraction > 200 μm) was significantly greater in the mixed treatment, and more C derived from the maize was associated with silt- and clay-sized particles. A two-component model fitted to the data yielded a rapidly mineralizable C pool (about 20% of total C) and a slowly mineralizable pool (about 80%). Generally, the size of the rapidly mineralizable C pool was rather small because inorganic N was rapidly immobilized after the addition of maize. However, the different mean half-lives of the C pools (rapidly decomposable mixed 0.035 years, and surface-applied 0.085 years; slowly decomposable mixed 0.96 years, and surface-applied 1.7 years) showed that mineralization was delayed when the straw was left on the surface. This seems to be because there is little contact between the soil microflora and plant residues. Evidently, the organic matter is more decomposed and protected within soil inorganic compounds when mixed into the soil than when applied on the soil surface, despite similar rates of mineralization.