Influence of tillage, crop rotation and nitrogen fertilization on soil organic matter and nitrogen under rain-fed Mediterranean conditions

The long-term effects of cropping systems and management practices on soil properties provide essential information for assessing sustainability and environmental impact. Field experiments were undertaken in southern Spain to evaluate the long-term effects of tillage, crop rotation and nitrogen (N) fertilization on the organic matter (OM) and mineral nitrogen (N_min) contents of soil in a rain-fed Mediterranean agricultural system over a 6-year period. Tillage treatments included no tillage (NT) and conventional tillage (CT), crop rotations were of 2 yr with wheat (Triticum aestivum L.)-sunflower (Helianthus annuus L.) (WS), wheat-chickpea (Cicer arietinum L.) (WP), wheat-faba bean (Vicia faba L.) (WB), wheat-fallow (WF), and in addition, continuous wheat (CW). Nitrogen fertilizer rates were 50, 100, and 150 kg N ha⁻¹. A split-split plot design with four replications was used. Soil samples were collected from a depth of 90 cm at the beginning of the experiment and 6 yr later. Soil samples were also collected from a depth of 30 cm after 4 yr. These samples, like those obtained at the beginning of the experiment, were subjected to comprehensive physico-chemical analyses. The soil samples that were collected 6 yr later were analyzed for OM, NH₄⁺---N and NO₃⁻---N at the 0–30, 30–60 and 60–90 cm soil depths. The tillage method did not influence the OM or N_min contents of the soil, nor did legume rotations increase the OM content of soil relative to CW. A longer period may have been required for differences between treatments to be observed owing to the small amount of crop residue that is returned to soil under rain-fed conditions of semi-arid climates. The WF rotation did not raise the N_min content of the soil relative to the other rotations. The consistent significant interaction between tillage and crop rotation testifies to the differential effect of the management system on the OM content and N status of the soil. The ammonium levels clearly exceeded those of NO₃⁻---N throughout the soil profile. The high N_min content of the soils reveals the presence of abundant N resources that should be borne in mind in establishing N fertilization schemes for crops under highly variable climatic conditions including scant rainfall such as those of the Mediterranean region.     

Influence of organic matter and inorganic fertilizer on the growth and nitrogen accumulation of corn plants

Effects of different nitrogen (N) sources on the growth and N accumulation of corn plants were studied on plots treated with a compost, a leguminous green manure, and a peat, respectively, associated with a chemical N fertilizer. The experiment included seven treatments with a no‐fertilization check and a conventional chemical fertilizer treatment. Whole corn plants were sampled, and total N was analyzed at 22, 33, 56, 77, and 120 days after seeding (DAS). The results showed that compost with an adequate amount of chemical N fertilizer could reach a high dry matter yield and a high N accumulation, even higher than those of the conventional chemical N fertilizer treatment. With green manure, a considerable amount of N was mineralized and utilized by the corn plants for growth and resulted in a good yield. Neither the peat nor the compost alone could supply enough N for the growth of corn plants. There were no significant effect of treatments on the distribution of dry matter yield and N accumulated in various organs. The crop growth rate of the corn plants of different treatments were significantly different at the vegetative growth stage, however, there were no significant difference during the grain filling period. The apparent N recovery of various treatments were between 0.22 to 0.51 kg N for each kg N applied.     

Influence of soil compaction on carbon and nitrogen mineralization of soil organic matter and crop residues

 We studied the influence of soil compaction in a loamy sand soil on C and N mineralization and nitrification of soil organic matter and added crop residues. Samples of unamended soil, and soil amended with leek residues, at six bulk densities ranging from 1.2 to 1.6 Mg m^–3 and 75% field capacity, were incubated. In the unamended soil, bulk density within the range studied did not influence any measure of microbial activity significantly. A small (but insignificant) decrease in nitrification rate at the highest bulk density was the only evidence for possible effects of compaction on microbial activity. In the amended soil the amounts of mineralized N at the end of the incubation were equal at all bulk densities, but first-order N mineralization rates tended to increase with increasing compaction, although the increase was not significant. Nitrification in the amended soils was more affected by compaction, and NO₃ ^–-N contents after 3 weeks of incubation at bulk densities of 1.5 and 1.6 Mg m^–3 were significantly lower (by about 8% and 16% of total added N, respectively), than those of the less compacted treatments. The C mineralization rate was strongly depressed at a bulk density of 1.6 Mg m^–3, compared with the other treatments. The depression of C mineralization in compacted soils can lead to higher organic matter accumulation. Since N mineralization was not affected by compaction (within the range used here) the accumulated organic matter would have had higher C : N ratios than in the uncompacted soils, and hence would have been of a lower quality. In general, increasing soil compaction in this soil, starting at a bulk density of 1.5 Mg m^–3, will affect some microbially driven processes. Received: 10 June 1999     

Net nitrogen mineralization from light- and heavy-fraction forest soil organic matter

Fine surface soil ( < 2 mm) from four sites in Oregon and Washington and three in Costa Rica was separated by repeated notation in NaI solution (sp. gr. < 1.2, 1.4, or 1.6 g cm^?3) into a light and a heavy fraction. Most organic matter in the light fractions consisted of partly-decomposed root fragments and other plant and microbial remnants and most in the heavy fractions was adsorbed or deposited on mineral surfaces or was protected within organo-mineral microaggregates. The light fraction had a consistently wider C:N ratio than the heavy, and net N mineralization during anaerobic incubation was greater from the heavy than from the light fraction in five of six soils for which both fractions were incubated. Net N mineralization was greater from the heavy fraction than from the whole soil of most sites perhaps because the light fraction immobilized N released from the heavy fraction when they were incubated together. Correlation between net N mineralization (as a proportion of total N) and C:N ratio was negative for the light fraction (r²=0.74) but positive for the heavy fraction (r² = 0.85), suggesting that the C:N ratio does not control the extent to which heavy-fraction N is mineralizable.     

Nitrate leaching: modifying the loss from mineralized organic matter

The impact on nitrate leaching of agronomic practices designed to immobilize nitrogen in autumn and winter was investigated over 4 years. Experimental treatments (reducing tillage depth, incorporating harvest residues, reducing fertilizer N by growing unfertilized grass or by spring-sown rather than autumn-sown crops) were compared with a control treatment in which autumn crops were sown after burning harvest residues, mouldboard ploughing and seedbed preparation. Winter cover cropping was also compared with winter fallowing. In the first year, incorporation of harvest residues or reducing tillage depth significantly decreased nitrate leaching compared with the control. Unfertilized grass did not affect leaching in the first winter but significantly decreased it in years 2 and 3. When winter cover crops were grown, nitrate leaching was never less than that under an autumn-sown cereal, and in the subsequent year leaching could be significantly greater. Winter fallowing caused the most nitrate leaching over the year. In the winter following a spring-sown crop, leaching under an autumn-sown crop greatly increased. Summed over 4 years, most leaching occurred with the winter fallow—spring cropping treatment; it was 18% more than where a winter cover crop preceded the spring crop. Reducing tillage depth or incorporating harvest residues did not significantly decrease leaching. Unfertilized grass ley followed by an autumn-sown cereal in the fourth year was the only treatment that significantly decrease leaching. Unfertilized grass ley followed by an autumn-sown cereal in the fourth year was the only treatment that significantly reduced leaching loss compared with the control. Incorporating harvest residues resulted in a balance between annual N inputs and outputs. All other treatments required substantial net annual N mineralization to balance annual inputs and outputs.     

Quantifying soil water effects on nitrogen mineralization from soil organic matter and from fresh crop residues

A loamy sand was incubated with and without addition of carrot leaves at six different water contents ranging from 6% to 20% (g 100 g^-1 dry soil) and N mineralization was monitored during 98 days. We calculated zero- and first-order rates for mineralization in the unamended soil and first-order rates for N mineralization in the residue-amended soil. Although N mineralization was strongly affected by soil moisture, rates were still important at 6% water content (corresponding to permanent wilting point), particularly in the residue-amended soil. Soil water content was recalculated as soil water tension and as percent water-filled pore space (%WFPS) and a parabolic, a logistic and a Gaussian-type function were fitted to the relation between N mineralization rates and water content, %WFPS or pF. Water potential was a less suitable parameter than either %WFPS or water content to describe the soil water influence on N mineralization, because N mineralization rates were extremely sensitive to changes in the water potential in the range of pF values between 1.5 and 2.5. In the residue-amended soil the Gaussian model yielded an optimum %WFPS of 56% for N mineralization, which is slightly lower than optimum values cited in literature. N mineralization in the unamended soil was more influenced by soil water than N mineralization from fresh crop residues. This could be explained by less water limitation of the microbial population decomposing the residues, due to the water content of the residues. The effect of the water contained in the residues was most pronounced in the lowest water content treatments. The water retention curves of both undisturbed and repacked soil were determined and suggested that extrapolation of results obtained during laboratory incubations, using disturbed soil, to field conditions will be difficult unless soil bulk density effects are accounted for, as is the case with the use of %WFPS.     

Influence of foliar phosphorus and nitrogen contents on chemical properties of water extractable organic matter derived from fresh and decomposed sugar maple leaves

The effects of watershed-scale experimental acidification on the macronutrient content and decomposition of sugar maple (Acer saccharum Marsh) leaves were investigated. Bear Brook Watershed in Maine (BBWM) is a paired forest watershed study where the West Bear (WB) watershed has been treated bi-monthly with 1800 eq ha^?1 yr^?1 of (NH₄)₂SO₄ since 1989, and the adjacent East Bear (EB) watershed has acted as a reference. Leaf samples collected from the treated WB watershed had significantly higher concentrations of N and P than leaves from the reference EB watershed. Leaves from both watersheds were decomposed for a 10-day laboratory incubation. Extractable total soluble carbon (C_TS) content of the leaves decreased following decomposition to a greater extent in WB leaves than in EB leaves. Spectroscopic and chromatographic chemical analyses indicated similar chemical properties for the fresh WB and EB WEOM. However, after decomposition, the WB WEOM was more humified as compared to EB WEOM indicating that the watershed treatment resulted in leaves which were more biodegradable than those in the reference watershed. Multi-dimensional fluorescence spectroscopy with parallel factor analysis (PARAFAC) modeled five components: tyrosine-like, three humic substance-like, and terrestrial/anthropogenic associated-like fluorophores. Following decomposition, the relative concentrations of two of the humic-associated components increased to a significantly greater extent for WB than for EB WEOM. These observations were consistent with greater decomposition-related changes to the WEOM from WB samples relative to EB samples. Pearson correlation analysis showed that foliar N and P concentrations were positively correlated with indices of humification. Adsorption of WEOM to goethite and gibbsite was significantly greater for decomposed WB WEOM than EB WEOM. These results demonstrate that greater leaf N and P contents can increase short-term decomposition, accelerate production of more humic-like WEOM, and thereby potentially influence the distribution of organic matter within the soil carbon pool.     

Influence of metals on solubility of soil organic matter

Treatment of the alkali-soluble organic matter of soil with a cation-exchange resin resin (Amberlite IR 120, H⁺ form) strongly modified the solubility characteristics of the organic matter, even though only part of the metals was removed. Two or more types of sorption sites were involved in the binding of metals by soil organic matter. The Amberlite removable metals interfered with the separation of humic from fulvic acid.     

Recovery of soil organic matter,organic matter turnover and nitrogen cycling in a post-mining forest rehabilitation chronosequence

Recovery of soil organic matter, organic matter turnover and mineral nutrient cycling is critical to the success of rehabilitation schemes following major ecosystem disturbance. We investigated successional changes in soil nutrient contents, microbial biomass and activity, C utilisation efficiency and N cycling dynamics in a chronosequence of seven ages (between 0 and 26 years old) of jarrah (Eucalyptus marginata) forest rehabilitation that had been previously mined for bauxite. Recovery was assessed by comparison of rehabilitation soils to non-mined jarrah forest references sites. Mining operations resulted in significant losses of soil total C and N, microbial biomass C and microbial quotients. Organic matter quantity recovered within the rehabilitation chronosequence soils to a level comparable to that of non-mined forest soil. Recovery of soil N was faster than soil C and recovery of microbial and soluble organic C and N fractions was faster than total soil C and N. The recovery of soil organic matter and changes to soil pH displayed distinct spatial heterogeneity due to the surface micro-topography (mounds and furrows) created by contour ripping of rehabilitation sites. Decreases in the metabolic quotient with rehabilitation age conformed to conceptual models of ecosystem energetics during succession but may have been more indicative of decreasing C availability than increased metabolic efficiency. Net ammonification and nitrification rates suggested that the low organic C environment in mound soils may favour autotrophic nitrifier populations, but the production of nitrate (NO₃^?) was limited by the low gross N ammonification rates (≤1 μg N g^?1 d^?1). Gross N transformation rates in furrow soils suggested that the capacity to immobilise N was closely coupled to the capacity to mineralise N, suggesting NO₃^? accumulation in situ is unlikely. The C:N ratio of the older rehabilitation soils was significantly lower than that of the non-mined forest soils. However, variation in ammonification rates was best explained by C and N quantity rather than C:N ratios of whole soil or soluble organic matter fractions. We conclude that the rehabilitated ecosystems are developing a conservative N cycle as displayed by non-mined jarrah forests. However, further investigation into the control of nitrification dynamics, particularly in the event of further ecosystem disturbance, is warranted.     

Influence of zinc on aerobic and anaerobic decomposition of organic matter

Increasing amounts of Zn were added to organic manure materials in order to establish its permissible maximum concentrations depending on the decomposition process and Zn-source. The mixtures were allowed to decompose aerobically for 46 days and anaerobically for 52 days, respectively. Zn-concentrations up to 2 000 ppm as ZnSO₄ and ZnO or 50 mg/l as ZnCl₂ did not significantly decrease the overall decomposition processes, although 50 mg/l as ZnCl₂ already delayed the anaerobic bio-gas formation. However, 6 000 ppm Zn as ZnSO₄ or 10 000 ppm as ZnO inhibited the aerobic decomposition by 25 %, whereas 100 mg/l Zn as ZnCl₂ reduced the bio-gas production by 20% relative to the control. All higher concentrations strongly inhibited both aerobic and anaerobic decomposition, respectively.     

Properties of dissolved organic matter related to soil organic matter quality and nitrogen additions in Norway spruce forest floors

The quality of dissolved organic matter (DOM) is highly variable and little information is available on the relation of DOM quality to the structure and composition of its parent soil organic matter (SOM). The effect of increasing N inputs to forest soils on the structure and composition of both SOM and DOM also remains largely unclear. Here we studied the release of DOM, its specific UV absorption and two humification indices (HIX) derived from fluorescence spectra from Oa material of 15 North- and Central-European Norway spruce (Picea abies (L.) Karst.) stands. The Oa material was incubated aerobically at 15 °C and water holding capacity over a period of 10 months and extracted monthly with an artificial throughfall solution. Soil respiration was determined weekly. The influence of mineral N inputs on composition of DOM and on respiration rates was investigated on periodically NH₄NO₃-treated Oa samples of eight selected sites. Release of dissolved organic carbon (DOC) from untreated Oa material samples ranged from 0.0 to 58.6 μg C day⁻¹ g C⁻¹ and increased with increasing C-to-N ratio. One HIX and UV absorption of DOM were negatively correlated to the degree of oxidation of lignin-derived compounds and positively to the C-to-N ratio and – HIX only – to the aromatic C content of SOM. Mineral N addition had no distinct effect on respiration rates. In six of eight samples the N-treatment caused an increase in specific UV absorption or one HIX of DOM. However, these effects were not statistically significant. Addition of mineral N did not affect the rates of DOM release. Our results show that properties of SOM largely determine the amount and quality of DOM in forest floors. Changes of DOM quality due to mineral N additions are likely, but we cannot confirm significant changes of DOM release.     

Influence of organic matter and pH on bentazone sorption in soils

Bentazone (3-isopropyl-1H-2,1,3-benzonthiadiazain-(4)3H-one 2,2-dioxide) is a postemergence herbicide which is used extensively worldwide, especially in China. The sorption of bentazone in various types of soils and extracted humic acids was investigated using a batch equilibration technique. Significant linearity was observed in sorption isotherms in five different types of soil, with distribution coefficients (K(d)) that varied between 0.140 and 0.321 mL g(-1). The distribution coefficient was determined to be a function of organic matter and pH in the soil. A model based on distribution coefficients was developed to predict bentazone sorption in soils. The organic matter-normalized partition coefficients for the neutral and anionic forms of bentazone were 370.3 and 2.40 mL g(-1), respectively. Hence, more attention should be given to the potential leaching problem when bentazone is applied in soils containing low organic matter and high pH.     

Rapid soil organic matter loss from forest dieback in a subalpine coniferous ecosystem

Forest dieback caused by climate-change associated stresses and insect outbreaks has emerged as a global concern, and the biogeochemical consequences of this phenomenon need to be elucidated. We measured biological and chemical traits of soil beneath live trees or trees recently killed by a mountain-pine-beetle outbreak in a subalpine coniferous forest in the Front Range of Colorado. We focused on the top 5 cm of mineral soil just beneath the O horizon and measured microbial biomass, soil invertebrate abundance and composition, and soil chemical characteristics. With the termination of inputs from rhizodeposition, mycorrhizal fungal turnover and fine root turnover, soil total carbon (C) and total nitrogen (N) in the mineral soil at three sites decreased by 38–49% and 26–45%, respectively. Tree mortality was associated with reduced soil microbial biomass but soil nematode and microarthropod densities were unchanged. Nematode trophic structure was altered with an increased proportion of bacterial feeders. Soil inorganic N concentrations were inversely correlated to microbial C:N ratios. Tree death was associated with increased soil pH, a possible loss of calcium (Ca²⁺), but an accumulation of soil inorganic N, largely as NH₄⁺. Our results suggest that forest dieback results in rapid C and N loss from surface mineral soils and that the accumulation of soil inorganic N, the reduction in microbial biomass, and the more bacterial-based soil food web increase the potential of enhanced N loss from affected ecosystems.     

Theoretical analyses of interactions between inorganic nitrogen and soil organic matter

The organic matter of the soil has a large potential to retain inorganic nitrogen by means of both biotic (microbially mediated) and abiotic (chemical) reactions. We derive one equation with which we analyse the nitrogen retention due to these mechanisms in terms of organic matter properties. We first of all show how to separate gross mineralization from immobilization. We then show that our equation can reproduce studies of microbially–mediated or purely inorganic immobilization. We then apply the theory to soils of different characteristics (typical agricultural and forest soils) and find that the major factor determining the fraction of added inorganic nitrogen that is immobilized is the ratio between soil carbon and inorganic nitrogen concentration.     

Evolution of organic matter and nitrogen during co-composting of olive mill wastewater with solid organic wastes

 Four olive mill wastewater (OMW) composts, prepared with three N-rich organic wastes and two different bulking agents, were studied in a pilot plant using the Rutgers system. Organic matter (OM) losses during composting followed a first-order kinetic equation in all the piles, the slowest being the OM mineralisation rate in the pile using maize straw (MS). The highest N losses through NH₃ volatilisation occurred in the mixtures which had a low initial C/N ratio and high pH values during the process. Such losses were reduced considerably when MS was used as the bulking agent instead of cotton waste (CW). N fixation activity increased during the bio-oxidative phase before falling during maturation. This N fixation capacity was higher in piles with a lower NH₄ ⁺-N concentration. Only the composts prepared with OMW, CW and poultry manure or sewage sludge reached water-soluble organic C (C_W) and NH₄ ⁺-N concentrations and C_W/N_org and NH₄ ⁺/NO₃ ^– ratios within the established limits which indicate a good degree of compost maturity. Increases in the cation-exchange capacity, the percentage of humic acid-like C and the polymerisation ratio revealed that the OM had been humified during composting. The germination index indicated the reduction of phytotoxicity during composting. Received: 14 June 1999     

Influence of the rhizosphere on microbial biomass and recently formed organic matter

We have aimed to quantify the effect of roots on the size of the soil microbial biomass, and their influence on the turnover of soil organic matter and on the extent of the rhizosphere. We sampled a sandy clay loam topsoil from two subplots with different treatment histories. One had a normal arable fertilization record, the other had received only inorganic nitrogen fertilizer but no phosphorus and potassium for 30 years. Glucose labelled with ¹⁴C was added to both samples which were then incubated for 4 weeks before the soil was packed in cylinders and planted with ryegrass. In both soils, microbial biomass at the root surface doubled during the first 8 days. At day 15, the microbial biomass had further increased in the fertile soil, and the rhizosphere effect had extended 2.5 mm into the fertile soil, but to only 1 mm in the infertile soil. The microbial ¹⁴C increased threefold near the roots in the fertile soil as a result of assimilation of previously formed microbial residues, but in the infertile soil there was no increase. There was a close relation between the increase in microbial ¹⁴C and a decrease in ¹⁴C soluble in 2 m KCl, indicating that the microbial residues were more weakly adsorbed in the fertile soil. We conclude that the increased microbial population living near the root surfaces re‐assimilated part of the ¹⁴C‐labelled microbial residues in the fertile soil. In the infertile soil, microbial residues resisted decomposition because they were more strongly sorbed on to soil surfaces.     

Denitrification loss from a grassland soil in the field receiving different rates of nitrogen as ammonium nitrate

Denitrification loss from a loam under a cut ryegrass sward receiving 0, 250 and 500 kg N ha^?1 a^?1 in four equal amounts was measured during 14 months using the acetylene-inhibition technique. The rate of denitrification responded rapidly to changes in soil water content as affected by rain. Mean rates of denitrification exceeded 0.2 kg N ha^?1 day^?1 only when the soil water content was >20% (w/w) and nitrate was >5μ N g^?1 in the upper 20 cm of the profile and when soil temperature at 2 cm was >5–8°C. When the soil dried to a water content <20%, denitrification decreased to <0.05 kg N ha^?1 day^?1. Highest rates (up to 2.0 kg N ha^?1 day^?1) were observed following application of fertilizer to soil at a water content of about 30% (w/w) in early spring. Denitrification in the control plot during this period was generally about a hundredth of that in plots treated with ammonium nitrate. High rates of N₂O loss (up to 0.30 kg N ha^?1 day-1) were invariably associated with high rates of denitrification (> 0.2 kg N ha^?1 day^?1). However, within 2–3 weeks following application of fertilizer to the plot receiving 250 kg N ha^?1 a^?1 the soil acted as a sink for atmospheric N₂O when its water content was >20% and its temperature >5–8°C. Annual N losses arising from denitrification were 1.6, 11.1 and 29.1 kg N ha^?1 for the plots receiving 0, 250 and 500 kg N ha^?1 a^?1, respectively. More than 60% of the annual loss occurred during a period of 8 weeks when fertilizer was applied to soil with a water content >20%.     

Effects of ammonium and nitrate on mineralization of nitrogen from leguminous residues

A laboratory incubation experiment was conducted to compare the effects of NH _inf4 ^sup+ and NO _inf3 ^sup- on mineralization of N from ¹⁵N-labelled vetch (Vicia villosa Rotn) in an Illinois Mollisol, and to determine the effect of a nitrification inhibitor (nitrapyrin) on mineralization of vetch N when used with NH _inf4 ^sup+ . The addition of either NH _inf4 ^sup+ or NO _inf3 ^sup- (100 and 200 mg N kg^-1 soil) significantly increased mineralization of vetch N during incubation for 40 days. The effect was greater with NH _inf4 ^sup+ than with NO _inf3 ^sup- , and a further increase occurred in the presence of nitrapyrin (10 mg kg^-1 soil). The addition of NO _inf3 ^sup- retarded the nitrification of NH _inf4 ^sup+ -N derived from vetch.     

Influence of moisture on the stability of soil organic matter and plant residues

The effect of three levels of soil moistening on the organic matter mineralization was assessed for three arable soils and wheat straw in the course of a 150-day-long incubation experiment. It was found that the intensity of the organic matter mineralization increased in parallel to soil moistening in the podzolized chernozem and dark-chestnut soil and remained stable in the gray forest soil, which was explained by the low content of easily mineralizable fractions of active organic matter in the latter soil. The mineralization of wheat straw depended on the soil moistening rather than on soil properties.