Nitrogen mineralization dynamics in grass monocultures

Although Wedin and Tilman (1990) observed large differences in in situ N mineralization among monocultures of five grass species, the mechanisms responsible were unclear. In this study, we found that the species did not change total soil C or N, and soil C: N ratio (range 12.9–14.1) was only slightly, but significantly, changed after four years. Nor did the species significantly affect the total amount of N mineralized (per g soil N) in year-long aerobic laboratory incubations. However, short-term N mineralization rates in the incubations (day 1–day 17) differed significantly among species and were significantly correlated with annual in situ mineralization. When pool sizes and turnover rates of potentially mineralizable N (N_o) were estimated, the best model treated N_o as two pools: a labile pool, which differed among species in size (N_l, range 2–3% of total N) and rate constant (h, range 0.04–0.26 wk^–1), and a larger recalcitrant pool with a constant mineralization rate across species. The rate constant of the labile pool (h) was highly correlated with annual in situ N mineralization (+0.96). Therefore, plant species need only change the dynamics of a small fraction of soil organic matter, in this case estimated to be less than 3%, to have large effects on overall system N dynamics.     

Fertilization strategy can affect the estimation of soil nitrogen mineralization potential with chemical methods

Aims

Our objective was to evaluate if the relationship between quick indices of mineralizable N and soil N mineralization potential (N_o) is affected by long-term fertilization with inorganic fertilizers (IF) and/or poultry litter (PL).

Methods

Samples from four long-term research sites that were fertilized with IF, IF+PL, or PL (>10 years) were aerobically incubated for 24 weeks to determine N_o. Soil N_o was then related to the results from the Illinois soil test analysis (ISNT), extractions with hot KCl, NaOH, or phosphate-borate (PB), and an anaerobic incubation (N_an).

Results

All evaluated chemical methods were correlated with N_o (R²?>?0.59). However, the models to describe the association between the chemical methods and N_o differed depending on the fertilizer treatment, due to the inability of chemical indices to estimate the N_o increase observed in some PL-amended soils. In contrast, the relationship between N_an and N_o was not affected by fertilization strategy.

Conclusions

Chemical indices can lead to the underestimation of soil N mineralization potential in PL-treated soils when the indices are calibrated in IF-amended soils, whereas the biological method N_an predicted N_o independently from the field’s fertilization history.

     

Kinetic parameters of nitrogen mineralization rates of leguminous crops incorporated into soil

Summary Fresh leguminous plant residues were incorporated into soil columns and incubated at 23°C for up to 20 weeks. The N released from specific fractions (foliage, stems, and roots) of each residue were monitored at specific time intervals. Relationships between organic carbon, total nitrogen, CN ratio, lipids, and lignin content of the plant materials and the cumulative amount of N mineralized in soil were investigated. Statistical analyses indicated that the rates of N mineralized were not significantly correlated with the organic C nor lipid content of the residues. However, the cumulative amount of N released was significantly correlated with the total N content of the plant material (r=0.93^***). The percentage of organic N of the legumes mineralized in soil ranged from 15.9 to 76.0%. The relationship between the percentage of N released and the CN ratio of the plant material showed an inverse cuvilinear response (r= 0.88^***). It was also evident that the composition of lignin in the residue influenced N mine-ralization rates of the leguminous organs incorporated into soil.There was a curvilinear relationship between the cumulative amount of N released from the residues and time of incubation. Nitrogen mineralization rates were described by first-order kinetics to estimate the N mineralization potential (N₀), mineralization rate constant (k), and the time of incubation required to mineralize one-half of N₀ (t_1/2). The kinetic parameters were calculated by both the linear least squares (LLS) and nonlinear least squares (NLLS) transformations. The N₀ values among the crop residues varied from –35 to 510 g Ng^–1 soil. Statistical analyses revealed that the N₀ values obtained by both LLS and NLLS methods were significantly correlated (r=0.93^***). The mineralization rate constants (k) of the residues ranged from 0.045 to 0.325 week^–1. The time of incubation required to mineralize one-half the nitrogen mineralization potential (t_1/2) of the legumes incorporated into soil ranged from 2.1 to 15.4 weeks.     

Determination ofin situ KN by the chloroform fumigation-incubation method and mineralization of biomass N under anaerobic conditions

A silt loam soil from Pakistan was incubated at 30°C with increasing levels (67, 133, 200, 267 and 333 μg N g^?1 soil) of¹⁵N-labelled (NH₄)₂SO₄ and glucose (C/N ratio of 30 for all additions). At a stage when all of the applied¹⁵N was immobilized (transformed into microbial biomass), moist soil samples were subjected to the chloroform fumigation-incubation method (CFIM) for determination of K_N and microbial biomass. Mineralization of biomass derived from the applied¹⁵N and the native soil N was studied under anaerobic conditions. In situ values of K_N varied from 0.19 to 0.42 and increased with increasing levels of amendment (N + glucose). From 10 to 18% of the native soil N was found as microbial biomass. Anaerobic incubation of the soils resulted in the mineralization (determined as NH ₄ ⁺ ) of 15.08 to 29.23% of the biomass¹⁵N at different levels of amendment; 2.90 to 4.43% of the native soil N was mineralized. From 70 to 90% of the N mineralized from native soil N was derived from microbial biomass; the rest was attributed to non-biomass N.     

Effect of legume-based cropping systems on nitrogen mineralization potential of Vertisol

The quantity and patterns of net mineralization of soil nitrogen (N) were studied in Vertisols under different cropping systems in the semi-arid tropical areas. Eight cropping systems were selected; three contained pigeonpea (PP), one contained PP and cowpea (COP), and two contained chickpea (CP) as legume component crops, one included sequence cropping with nonlegumes during the rainy and postrainy seasons, and one system was kept fallow (F) during the rainy season and sown to sorghum (S) during the postrainy season. Cropping systems with PP as a component crop increased mineralizable N(N _o ) content two-fold in the soil compared with fallow + sorghum (F+S)–F+S system. The N mineralization rate constant (k) was not significantly affected by previous cropping history of the soil; however, a numerically higher rate constant was observed in the COP/PP intercrop, followed by sequential S+safflower (SF) system as compared to the other soils. Mineral N accumulation curves for six soils were more accurately described by the exponential model than the linear model. The active N fraction (N _o /N_tot %) varied between 8 and 16% for different systems and a direct relationship was observed between N _o /N_tot and total N for the soils under diverse cropping systems.ICRISAT JA (1638)     

Nitrogen mineralization rates in salinevs. salt-amended soils

Studies were conducted to compare N mineralization rates in salt-amended nonsaline soils to naturally-occurring saline soils. NaCl, CaCl₂, and Na₂SO₄ were added to nonsaline soils at rates that produced electrical conductivities of the saturation extracts (EC_e) of 5, 10, 15, and 20 dS m⁻¹. Saline soils with similar properties were leached to the same EC_c levels. N mineralization in the Chino soil was inhibited by salt addition, particularly with sodium and calcium chlorides. In the Domino soil there was some inhibition of N mineralization with the chloride salts, but enhancement with Na₂SO₄ was observed. Nitrification in both soils was more sensitive to salt addition than ammonification. N mineralization occurred more slowly in both leached saline soils compared to the salt-amended soils. Leached saline soils often accumulated greater amounts of inorganic N compared to their native saline counterparts, particularly with the 5 dS m⁻¹ Chino soil (native, 44 dS m⁻¹) and with the 5, 10, 15 and 20 dS m⁻¹ Domino soils (native, 32 dS m⁻¹). Kinetic parameters were estimated by the linear least squares (LLS) and the nonlinear least squares (NLLS) methods. Generally, the LLS transformation estimated greater values of potentially mineralizable N (N_o) and lower rate constants (k). With the NLLS equation, N_o values for the leached saline soils were usually lower, and k values usually higher than in the salt-amended soils. The nonsaline controls generally had the highest N_o and lowest k estimates. Average LLS rate constants for the salt-amended and leached saline soils were 0.055 and 0.083 for the Chino, and 0.104 and 0.137 week⁻¹, respectively, for the Domino soils. With the NLLS equation, average k values for the salt-amended and leached saline soils were 0.087 and 0.089 for the Chino, and 0.181 and 0.387 week⁻¹, respectively, for the Domino soils. These results suggest that N mineralization rates obtained in salt-amended nonsaline soils may not be representative of those in naturally-occurring saline soils.     

Effects of clear-cutting and burning on characteristics of nitrogen mineralization and microbes in the forest soil of a Pinus massoniana plantation in Southern China

The aim of this study was to clarify the effects of clear-cutting and burning (CCB) on soil fertility in a Pinus massoniana (Masson pine) plantation after CCB in Fujian Province, China. We investigated changes in nitrogen (N) mineralization potential (N₀), N mineralization rate constant (k) and the apparent activation energy (Ea) of the soil, with a mathematical analysis using a kinetics model based on the results of in vitro incubation. Changes in the amount of microbial biomass nitrogen (MBN), as well as the number of heterotrophic and nitrite-oxidizing bacteria, were also investigated. The N₀, MBN and the number of fungi and actinomycetes in forest soil was reduced for at least 18 months after CCB. The number of heterotrophic and nitrite-oxidizing bacteria increased, and k, Ea and N mineralization became greater after 6 months of CCB, compared with the control plots. Because there were few young trees planted, which would have taken up mineralized N in the post-CCB site, it is probable that a high proportion of the mineralized N that accumulated in the soil may have been lost during the summer rainy season. Therefore, it is suggested that CCB led to a deficiency in available N during short rotations, which resulted in soil degradation.     

Soil nitrogen availability evaluations based on nitrogen mineralization potentials of soils and uptake of labeled and unlabeled nitrogen by plants

Summary Sudangrass [Sorghum sudanense (Piper) Stapf] was grown in a greenhouse pot experiment on 39 soils having a broad range of chemical and physical characteristics. Labelled N as sodium nitrate (9% excess N¹⁵) was applied at rates of 200 and 400 mg of N per pot (2kg of soil). After 6 weeks of growth, total N and N¹⁵ were determined on plant tops and roots and on the cropped soils. Maximum yield differed widely among the soils owing to variations in yield-limiting factors other than N. Despite the diversity of responses to N fertilizer, the experiment provided a meaningful basis for assessing soil nitrogen availability. Amounts of N taken up from soils were similar from pots receiving no fertilizer N and from pots receiving labeled N.Amounts of soil organic N mineralized during cropping plus the mineral N present initially in the soils correlated highly with amounts of soil N taken up by whole plants (tops and roots). Average recovery by whole plants of mineral N formed before and during the cropping period was about 85 per cent, a value corresponding closely to recovery of fertilizer N in this experiment. The similarity in recovery of N provided by soil and fertilizer suggests that mineral N from these sources comprised a common pool that behaved as an entity with respect to mineralization-immobilization relations or other reactions affecting N availability to plants.A-values, the amounts of soil N having an availability equivalent to that of applied fertilizer N, were similar for two levels of applied labeled N and for tops and whole plants. Moreover, A-values were similar to amounts of N mineralized before and during crop growth. This result is particularly significant, since amounts of N mineralized during crop growth were estimated from N mineralization potentials, taking into account the effects of temperature on the mineralization rate constant. Thus, the study provides preliminary evidence that the soil N mineralization potential offers a basis for reliably estimating amounts of soil N mineralized during selected periods of time under specified temperature regimes.     

Oat plant effects on net nitrogen mineralization

Living plants have been reported to stimulate, inhibit, or have no effect on net nitrogen mineralization in soil. A series of experiments were conducted to evaluate the influence of living oat plants Avena sativa on net N mineralization. Oat plants were grown in plastic cylinders containing soil, and net N mineralization was assessed by determining the N balance in these microcosms. Measured N inputs included N contained in the oat seeds and N₂ fixation. N losses by NH₃ volatilization and denitrification were also measured. We observed that in some soils net N mineralization was stimulated by as much as 81%, but in other soils there was no effect of living oat plants on net N mineralization. N mineralization responses are related to past cropping histories of the soils.     

Comparison between a chemical and a biological method to determine the N mineralization potential of temperate-humid region soils

The potentially mineralizable organic N of 33 different soils was estimated by a chemical test (hot extraction with 2N KCl) and the values compared with those previously obtained by a biological method (aerobic incubation in the laboratory). On average, the organic N solubilized by the chemical procedure was significantly lower than that mineralized by a two weeks aerobic incubation for all the soils as a whole. The same was true for soils developed over acid rocks and over sediments. However, the values obtained for the soils developed over limestone and basic rocks were similar by both methods. The values obtained by both methods were not significantly correlated neither when considering all soils together nor when considering different groups according to soil management or parent material. Significant correlations between both methods were only found when the soils were separated into two groups according to their organic N content: soils with less than 400 mg N 100 g^–1 soil and soils with more than 400 mg N 100 g^–1 soil. The organic N solubilized by the chemical procedure was significantly correlated with the hexosamine-N content; however, it was not correlated with the factors that control the biological mineralization of the organic N, except with the soluble Al content. Therefore, the chemical extraction did not seem to address the biologically active N pool in a selective way.     

Some factors affecting the mineralization of organic sulphur in soils

Summary Factors affecting the release of sulphate from a number of eastern Australian soils were studied.All of the soils released sulphate when dried. The amounts released were influenced by the manner in which the soil was dried. Air-drying in the laboratory at 20°C released least sulphate.Sulphate was mineralized in all soils by incubation at 30°C but the amounts mineralized could not be related to soil type or any single soil property. The ratio of nitrogen mineralized: sulphur mineralized varied widely between soils and was generally appreciably greater than the ratio of total nitrogen: organic sulphur in the soils.A rapid flush of mineralization of both sulphur and nitrogen took place when some of the soils were rewetted and incubated after they had been dried in the laboratory and stored for 4 to 5 months. Following this, the rate of mineralization was similar to that in the original undried soil. During this flush, the enhancement of sulphur mineralization was relatively greater than that of nitrogen so that the ratio of nitrogen mineralized: sulphur mineralized was considerably smaller than that during later phases of the incubation or that of the original moist soil. Soils collected after they had remained dry in the field for a similar period of time did not show this type of mineralization although they had initially done so when collected moist and air-dried in the laboratory.The effects of temperature, soil moisture, toluene and formaldehyde, and the addition of calcium carbonate to soils on the mineralization of sulphur were similar to their effects on the mineralization of nitrogen.     

Failure of an iterative curve-fitting procedure to successfully estimate two organic N pools

Fitting a double negative exponential function to N mineralization data can be used to characterize two organic nitrogen pools; an easily decomposable (N_dpm) and a resistant one (N_rpm). The relevance of those two calculated N mineralization pools was investigated by adding easily decomposable organic material to soils. Soil amended with crop residues of sugar-beet or bean was mixed with an equal amount of coarse sand, incubated at 35 °C and leached at specific time-intervals. Upon leaching, NH₄ ⁺ and NO₃ ^- were measured in the extracts. A double negative exponential function was fitted to the data and two organic N pools were defined. Fitting a double negative exponential function to N mineralization data to characterize an active and resistant organic N pool was sometimes impossible; the N mineralization data did not always resemble a negative exponential function. Additionally, the size of the two pools calculated were not constant with time and were often meaningless; the N_rpm pool was greater than the soil organic N content, the size of the N_rpm pool was smaller than the N_dpm pool or one of the N pools was negative. Relevant values for both N_rpm and N_dpm which were consistent with incubation time were only obtained when excessive amounts of organic material, normally not dealt with in the field, were applied.     

A global synthesis of the rate and temperature sensitivity of soil nitrogen mineralization: latitudinal patterns and mechanisms

Soil net nitrogen (N) mineralization (N_min) is a pivotal process in the global N cycle regulating the N availability of plant growth. Understanding the spatial patterns of N_min, its temperature sensitivity (Q₁₀) and regulatory mechanisms is critical for improving the management of soil nutrients. In this study, we evaluated 379 peer‐reviewed scientific papers to explore how N_min and the Q₁₀ of N_min varied among different ecosystems and regions at the global scale. The results showed that N_min varied significantly among different ecosystems with a global average of 2.41 mg N soil kg^?1 day^?1. Furthermore, N_min significantly decreased with increasing latitude and altitude. The Q₁₀ varied significantly among different ecosystems with a global average of 2.21, ranging from the highest found in forest soils (2.43) and the lowest found for grassland soils (1.67) and significantly increased with increasing latitude. Path analyses indicated that N_min was primarily affected by the content of soil organic carbon (C), soil C:N ratio, and clay content, where Q₁₀ was primarily influenced by the soil C:N ratio and soil pH. Furthermore, the activation energy (E_a) of soil N mineralization was significantly and negative correlated with the substrate quality index among all ecosystems, indicating the applicability of the carbon quality temperature hypothesis to soil N mineralization at a global scale. These findings provided empirical evidence supporting that soil N availability, under global warming scenarios, is expected to increase stronger in colder regions as compared with that low‐latitude regions due to the higher Q₁₀. This may alleviate the restriction of N supply for increased primary productivity at higher latitudes.     

Interactions between leaf litter and soil organic matter on carbon and nitrogen mineralization in six forest litter-soil systems

Background and aims

Leaf litter decomposes on the surface of soil in natural systems and element transfers between litter and soil are commonly found. However, how litter and soil organic matter (SOM) interact to influence decomposition rate and nitrogen (N) release remains unclear.

Methods

Leaf litter and mineral soil of top 0–5 cm from six forests were incubated separately, or together with litter on soil surface at 25 °C for 346 days. Litter N remaining and soil respiration rate were repeatedly measured during incubation. Litter carbon (C) and mass losses and mineral N concentrations in litter and soil were measured at the end of incubation.

Results

Net N transfer from soil to litter was found in all litters when incubated with soil. Litter incubated with soil lost more C than litter incubated alone after 346 days. For litters with initial C: N ratios lower than 52, net N_min after 346 days was 100 % higher when incubated with soil than when incubated alone. Litter net N_min rate was negatively related to initial C: N ratio when incubated with soil but not when incubated alone. Soil respiration rate and net N_min rate did not differ between soil incubated with litter and soil incubated alone.

Conclusions

We conclude that soils may enhance litter decomposition rate by net N transfer from soil to litter. Our results together with studies on litter mixture decomposition suggest that net N transfer between decomposing organic matter with different N status may be common and may significantly influence decomposition and N release. The low net N_min rate during litter decomposition along with the small size of litter N pool compared to soil N pool suggest that SOM rather than decomposing litter is the major contributor to plant mineral N supply.     

滇西北高原纳帕海湿地土壤氮矿化特征

采用树脂芯原位培育法,研究了纳帕海沼泽、沼泽化草甸和草甸土壤氮的矿化特征。结果表明,铵态氮（NH₄⁺-N）为沼泽、沼泽化草甸土壤中无机氮的主要存在形式,分别占无机氮含量的96.76%和75.24%,而硝态氮（NO₃^--N）为草甸土壤中无机氮的主要存在形式,占无机氮含量的58.77%。植物生长期内,纳帕海湿地土壤的净氮矿化速率表现为沼泽化草甸 > 草甸 > 沼泽,表明干湿交替的土壤环境更利于土壤氮矿化作用的进行,土壤中氮素有效性和维持植物可利用氮素的能力更强。整个生长季,沼泽和草甸土壤氮矿化为硝化作用,而沼泽化草甸土壤氮矿化为氨化作用。土壤硝态氮含量、有机质含量、碳氮比和含水量均对纳帕海沼泽、沼泽化草甸和草甸土壤的氮矿化产生显著影响。     

Mineralization of labeled N from cowpea [Vigna unguiculata (L.) Walp.] plant parts at two growth stages in sandy soil

Cowpea [Vigna unguiculata (L). Walp.] has great potential as green manure due to its rapid N accumulation and efficient N₂ fixation. The objective of this study was to measure the rate of N mineralization from cowpea plant parts harvested at onset of flowering (5 weeks) and mid pod-fill (7 weeks) under near optimum conditions. Cowpeas were grown in a greenhouse and supplied with ¹⁵NH₄ ¹⁵NO₃ to isotopically label tissue. Cowpea leaves, stems, and roots were incorporated into a sandy soil (Psammentic Paleustalf) and net N mineralized was measured several times during a 10 week incubation. The amount of N accumulated in 7-week old cowpeas was more than double that in 5-week old cowpeas. The portion of N mineralized after 10 weeks was 24% for 5-week old cowpeas and 27% for 7-week old cowpeas. The rate of N mineralization from leaves and stems increased with plant age, but decreased for roots. The amount of N mineralized from 7-week old cowpeas was more than double (235%) that from 5-week old cowpeas due to greater N accumulation and a more rapid rate of N mineralization of the more mature cowpeas. The greatest amount of N was released from leaves, which amounted to 74 and 65% of total N mineralization from 5- and 7-week old cowpeas, respectively. The percentage of N mineralized by 10 weeks was linearly related to the tissue N concentration of the plant parts and to their C/N ratio. These relationships allow a quick estimation of the amount of N that would mineralize from cowpea residues incorporated into soil based on their N concentration or C/N ratio.     

Effect of mild drying on the mineralization of soil nitrogen

Summary Drying soil to –100 kPa increased the subsequent mineralization of nitrogen under optimal moisture conditions. The effect was greater when the soils were dried to –1500 Pa. Mineralization was greater after four cycles of wetting and drying than after one. Depending on the drying conditions, the amount of nitrogen mineralized after drying to –1500 Pa was between 6.8 and 18.2% of that mineralized after chloroform fumigation. After drying the soils the average ratio of CO₂-C respired to min N was 21.1–22.3 depending on the drying conditions, whereas after chloroform treatment and autoclaving the ratio was 6.0 and 9.9 respectively. The effect of drying on nitrogen mineralization is attributed to two causes: the death and subsequent lysis of a small proportion of the soil organisms, and to the desorption of organic substances with a wide C/N ratio.Because of the stimulation of even mild drying conditions, marked differences in mineralization rates of soil nitrogen between cropping seasons must be expected.     

Soil organic matter quality influences mineralization and GHG emissions in cryosols: a field‐based study of sub‐ to high Arctic

Arctic soils store large amounts of labile soil organic matter (SOM) and several studies have suggested that SOM characteristics may explain variations in SOM cycling rates across Arctic landscapes and Arctic ecosystems. The objective of this study was to investigate the influence of routinely measured soil properties and SOM characteristics on soil gross N mineralization and soil GHG emissions at the landscape scale. This study was carried out in three Canadian Arctic ecosystems: Sub‐Arctic (Churchill, MB), Low‐Arctic (Daring Lake, NWT), and High‐Arctic (Truelove Lowlands, NU). The landscapes were divided into five landform units: (1) upper slope, (2) back slope, (3) lower slope, (4) hummock, and (5) interhummock, which represented a great diversity of Static and Turbic Cryosolic soils including Brunisolic, Gleysolic, and Organic subgroups. Soil gross N mineralization was measured using the ¹⁵N dilution technique, whereas soil GHG emissions (N₂O, CH₄, and CO₂) were measured using a multicomponent Fourier transform infrared gas analyzer. Soil organic matter characteristics were determined by (1) water‐extractable organic matter, (2) density fractionation of SOM, and (3) solid‐state CPMAS ¹³C nuclear magnetic resonance (NMR) spectroscopy. Results showed that gross N mineralization, N₂O, and CO₂ emissions were affected by SOM quantity and SOM characteristics. Soil moisture, soil organic carbon (SOC), light fraction (LF) of SOM, and O‐Alkyl‐C to Aromatic‐C ratio positively influenced gross N mineralization, N₂O and CO₂ emissions, whereas the relative proportion of Aromatic‐C negatively influenced those N and C cycling processes. Relationships between SOM characteristics and CH₄ emissions were not significant throughout all Arctic ecosystems. Furthermore, results showed that lower slope and interhummock areas store relatively more labile C than upper and back slope locations. These results are particularly important because they can be used to produce better models that evaluate SOM stocks and dynamics under several climate scenarios and across Arctic landscapes and ecosystems.     

Symbiotic N2-fixation by the cover crop Pueraria phaseoloides as influenced by litter mineralization

The perennial legume Pueraria phaseoloides is widely used as a cover crop in rubber and oil palm plantations. However, very little knowledge exists on the effect of litter mineralization from P. phaseoloides on its symbiotic N₂-fixation. The contribution from symbiotic N₂-fixation (Ndfa) and litter N (Ndfl) to total plant N in P. phaseoloides was determined in a pot experiment using a ¹⁵N cross-labeling technique. For determination of N₂-fixation the non-fixing plant Axonopus compressus was used as a reference. The experiment was carried out in a growth chamber during 9 weeks with a sandy soil and 4 rates of ground litter (C/N=16,2.8% N). P. phaseoloides plants supplied with the highest amount of litter produced 26% more dry matter and fixed 23% more N than plants grown in soil with no litter application, but the percentage of Ndfa decreased slightly, but significantly, from 87 to 84%. The litter N uptake was directly proportional to the rate of application and constituted 10% of total plant N at the highest application rate. Additionally, a positive correlation was found between litter N uptake and the amount of fixed N₂. The total recovery of litter N in plants averaged 26% at harvest (shoot + root) and was not affected by the quantity added. A parallel incubation experiment also showed that, as an average of all litter levels, 26% of the litter N was present in the inorganic N pool. The amounts of fertilizer and soil N taken up by plants decreased with litter application, probably due to microbial immobilization and denitrification. It is concluded that, within the litter levels studied, litter mineralization will result in a higher amount of N₂-fixed by P. phaseoloides.     

Indigenous legumes biomass quality and influence on C and N mineralization under indigenous legume fallow systems

Non-cultivated N₂-fixing indigenous legumes can be harnessed to enhance soil fertility replenishment of smallholder farms. Understanding N release patterns of biomass generated by such legumes is key in managing N availability to crops. Nitrogen and C mineralization patterns of indigenous legume species, mainly ofTephrosia andCrotalaria genera, and of soils sampled at termination of 1- and 2-year indigenous legume fallows (indifallows)were investigated in leaching tube incubations under laboratory conditions. With the exception ofTephrosia longipes Meisn (2.4%) andCrotalaria cylindrostachys Welw.ex Baker (1.8%), all indigenous legumes had >2.5% N. Total polyphenols and lignin were <4% and 15%, respectively, for all species.Crotalaria pallida (L.) andEriosema ellipticum Welw.ex Baker mineralized >50% of the added N in the first 30 days of incubation. Similar to mixed plant biomass from natural weed fallow,C. Cylindrostachys immobilized N during the 155-day incubation period. Indifallow fallow biomass reached peak N mineralization 55 days after most legumes had leveled off. Carbon release by legume species closely followedN release patterns,with mostCrotalaria species releasing >500 mg CO₂-C kg^?1 soil. Soils sampled at termination of fallows reached peak N mineralization in the first 21 days of incubation, with indifallows mineralizing significantly (P<0.05) more N than natural fallows. Application of mineral P fertilizer to indifallows and natural fallows increased C and N mineralization relative to control treatments. It was concluded that (i) indigenous legumes generate biomass of high quality within a single growing season, (ii) the slow N release of biomass generated under indifallow systems suggests that such fallows can potentially be manipulated to enhance N availability to crops, and (iii) N and C mineralization of organic materials in sandy soils is likely controlled by availability of P to the soil microbial pool.