Bare Fallow Soil Research Articles

Quantifying and isolating stable soil organic carbon using long-term bare fallow experiments

Abstract. The stability of soil organic matter (SOM) is a major source of uncertainty in predicting atmospheric CO2 concentration during the 21st century. Isolating the stable soil carbon (C) from other, more labile, C fractions in soil is of prime importance for calibrating soil C simulation models, and gaining insights into the mechanisms that lead to soil C stability. Long-term experiments with continuous bare fallow (vegetation-free) treatments in which the decay of soil C is monitored for decades after all inputs of C have stopped, provide a unique opportunity to assess the quantity of stable soil C. We analyzed data from six bare fallow experiments of long-duration (>30 yrs), covering a range of soil types and climate conditions, and sited at Askov (Denmark), Grignon and Versailles (France), Kursk (Russia), Rothamsted (UK), and Ultuna (Sweden). A conceptual three pool model dividing soil C into a labile pool (turnover time of a several years), an intermediate pool (turnover time of a several decades) and a stable pool (turnover time of a several centuries or more) fits well with the long term C decline observed in the bare fallow soils. The estimate of stable C ranged from 2.7 g C kg−1 at Rothamsted to 6.8 g C kg−1 at Grignon. The uncertainty associated with estimates of the stable pool was large due to the short duration of the fallow treatments relative to the turnover time of stable soil C. At Versailles, where there is least uncertainty associated with the determination of a stable pool, the soil contains predominantly stable C after 80 years of continuous bare fallow. Such a site represents a unique research platform for characterization of the nature of stable SOM and its vulnerability to global change.

Soil aggregation, aggregate stability, organic carbon and nitrogen in different soil aggregate fractions under forest and shrub vegetation on the Loess Plateau, China

Revegetation has been reported as one of the most effective counter measures to reduce soil and water erosion on the Loess plateau in China. Soil aggregate stability and the distribution of organic carbon and nitrogen in different aggregate fractions would be affected by different plant communities. The objectives of this study were to elucidate the effects of different plant communities on soil aggregate stability and the distribution of organic carbon and nitrogen in different aggregate fractions in order to prove that the different plant covers enhance soil aggregate stability. Six kinds of soil samples under forest (Quercus liaotungensis, Populus davidiana, Pinus tabulaeformis, Bothriochloa, a 14 year abandoned land, and a 19 year bare fallow soil. Four kinds of soil samples under shrub land (the 24 year old Caragana Korshinskii Kom.; the 14 year old C. Korshinskii Kom., 3 year old abandoned grazing land and traditional slope cropland which is claimed by the farmers for production with very low fertilizers) were collected from the hilly–gully area on the Loess Plateau, which was divided into 0–10 cm and 10–20 cm. We investigated soil aggregate stability and soil aggregate fractions by ultrasonic fractionation (USAS), and the distribution of organic carbon and nitrogen in different fractions under forest and afforested land, as key indicators for soil remediation through revegetation. The results showed that soil organic carbon (Corg) and total nitrogen (Nt) were strongly increased under forest and artificial shrub land compared to cropland and bare fallow land and were higher in the surface layers (0–10 cm) than in the subsurface (10–20 cm). Soil aggregate stability (SAS) was quite low under bare fallow land and cropping land soils, in comparison with the 4 forest communities. The three main fractions of soil aggregates, obtained by ultrasonic fractionation, were < 63 μm, 63–100 μm and 100–250 μm, which represented approximately 60%, 10% and 10%, respectively. In all land uses, macro-aggregates, 1000–630 μm > 630–250 μm had a higher Corg content than micro-aggregates, 250–100 μm > 100–63 μm ≫ 63 μm. In comparison to cropland and bare fallow land, forest and artificial shrub can protect and enhance the Corg, Nt and soil aggregate stability. The higher C:N ratio of macro-aggregate fractions indicates a rapid turnover of soil organic carbon in the top soil of forest sites and even on bare fallow and crop land. The narrow range of C:N ratio in micro-aggregates indicates that soil organic carbon in micro-aggregates is more stable than that in the macro-aggregates. We concluded that revegetation of eroded soils accelerates soil remediation and rehabilitation.

Rates of sheet and rill erosion in Germany — A meta-analysis

Knowledge of erosion rates under real conditions is of great concern regarding sustainability of landuse and off-site effects on water bodies and settlements. Experimentally derived rates of sheet and rill erosion are often biased by experimental settings, which deviate considerably from typical landuse, by short measuring periods and by small spatial extensions, which do not account for the pronounced spatio-temporal variability of erosion events. We compiled data from 27 studies covering 1076 plot years to account for this variability. Modelling was used to correct for deficiencies in the experimental settings, which overrepresented arable land and used steeper and shorter slopes as well as higher erosivity than typically found in reality. For example, the average slope gradient was 5.9° for all arable plot experiments while it is only 2.6° on total arable land in Germany. The expected soil loss by sheet and rill erosion in Germany after taking real slopes, landuse and erosivity into account averaged 2.7 t ha − 1 yr − 1 . Annual crops contributed the largest proportion (90%) but hops despite its negligible contribution to landuse (0.06%) still contribute 1.0% due to its extraordinary rapid erosion, which was even faster than the measured bare fallow soil loss standardized to otherwise identical conditions. Bare fallow soil loss, which is often used as baseline, was 80 t ha − 1 yr − 1 when standardized to 5.1° slope gradient, 200 m flow path length, and average German erosivity.

The Effects of Biogeotextiles on the Stabilization of Roadside Slopes in Lithuania

Biogeotextiles constructed from the leaves of Borassus aethiopum and Mauritia flexuosa are investigated at the Kaltinėnai Research Station of the Lithuanian Institute of Agriculture, which is participating in the EU-funded BORASSUS Project. Biogeotextiles are potentially excellent biodegradable and environmentally-friendly materials useful for soil conservation. Field studies on a steep (21–25°) roadside slope in Lithuania suggest biogeotextile mats are an effective and sustainable soil conservation technique. Biogeotextiles have a potential as a biotechnical soil conservation method for slope stabilization and protection from water erosion on steep industrial slopes and may be integrated with the use of perennial grasses to optimize protection from water erosion. The investigations demonstrated that a cover of Borassus and Buriti mats improved the germination and growth of sown perennial grasses. The biomass of perennial grasses increased by 52.0–63.4% under cover of Borassus mats and by 18.6–28.2% under cover of Buriti mats. Over 2 years, the biogeotextiles (Borassus and Buruti, respectively) decreased soil losses from bare fallow soil by 90.8% and 81.5% and from plots covered by perennial grasses by 87.9% and 79.0%, respectively.

Amino acid 15N in long‐term bare fallow soils: influence of annual N fertilizer and manure applications

SummaryLong‐term dynamics of amino acids (AAs), from a bare fallow soil experiment (established in 1928 at INRA‐Versailles, France), were examined in unamended control (Con) plots and plots treated with ammonium sulphate (Amsul), ammonium nitrate (Amnit), sodium nitrate (Nanit) or with animal manure (Man). Topsoil (0–25 cm) from 1929, 1963 and 1997 was analysed for C, N and 15N content and distribution of 18 amino acids recovered after acid hydrolysis with 6 m HCl. With time, soil N, C and AA content were reduced in Con, Amsul, Amnit and Nanit, but increased in Man. However, the absolute N loss was 3–11 times larger in Man than Nanit, Amsul, Amnit and Con, due to the much higher N annual inputs applied to Man. From 1929 to 1997 in Con, Amsul, Amnit and Nanit the whole soil and non‐hydrolysable‐N pool δ15N increased associated with the loss of N (indicative of Rayleigh 15N/14N fractionation). No δ15N change from 1929 to 1997 was found in the hydrolysable AA‐N (HAN) pool. Fertilizer N inputs aided stabilization of soil AA‐N, as AA half‐life in the mineral N fertilizer treatments increased from 34 years in 1963 to 50 years in 1997. The δ15N values of alanine and leucine reflected both source input and 15N/14N fractionation effects in soils. The δ15N increase of ornithine (∼6‰) was similar to the whole soil. The δ15N change of phenylalanine in Con (decrease of 7‰) was related to its proportional loss since 1929, whereas for Amsul, Amnit, Nanit and Man it was associated with isotope effects caused by the fertilizer inputs. However, the soil δ15N value of most individual amino acids (IAAs) did not significantly change over nearly 70 years, even with mineral or organic N inputs. We conclude for these bare fallow systems that: (i) δ15N changes in the whole soil and non‐hydrolysable AA pool were solely driven by microbial processes and not by the nature of fertilizer inputs, and (ii) without plant inputs, the δ15N of the HAN pool and (most) IAAs may reflect the influence of plant–soil interactions from the previous (arable cropping) rather than present (fallow) land use on these soil δ15N values.

Cover Crop Rotations Alter Soil Microbiology and Reduce Replant Disorders in Strawberry

In July 2001, a study was established in a field with a 30-year history of perennial strawberry production to examine effects on replant disorder of 12 different species of preplant cover crops, soil fumigation (methyl bromide plus chloropicrin), and fallow management. In May 2002, strawberries (`Jewel') were planted into pots containing soils with the incorporated cover crops, grown for 1 year, and then fruited. Strawberry yields in 2003 were highest in pots containing indiangrass (Sorghastrum avenaceum) and brown mustard (Brassica juncea) -incorporated soils, resulting in 32% and 28%, respectively, higher yield than plants in pots containing untreated, bare fallow soil. Yield was lowest in fumigated soil or soil incorporated with sunnhemp (Crotolaria juncea), having 19% and 10% less yield than the fallow treatment, respectively. In Aug. 1999, a complementary study was established in a field with a 7-year history of continuous perennial strawberry production to examine the effects of single species and multiple species rotations on replant disorder, bacterial populations, and fungal pathogens over 2 fruiting years. Cover crop treatments included various monocultures and sequences of perennial alfalfa (Medicago sativa), brown mustard, kale (B. oleracea `Winterbor'), sweet corn (Zea mays `Saccharata'), rye (Secale cereale), hairy vetch (Vicia villosa), marigold (Tagetes patula `Nema-gone'), oats (Avena sativa `Newdak'), and sudangrass (Sorghum bicolor × S. sudanese). These rotations were compared with the effects of fumigation using methyl bromide with chloropicrin (99:1), continuous strawberry, and bare fallow. Symptoms of replant disorder developed in the continuous strawberry plots within a few months of planting. Plants in the fumigation treatment produced greater fruit yield than all other treatments in 2003, 139% more than plants from the continuous strawberry treatment. Strawberry plants grown in the kale/sweet corn/rye treatment had consistently high yield, and both the hairy vetch/marigold/rye and the oats/sudangrass/rye treatments led to marked improvement over the continuous strawberry treatment. Plants from the brown mustard treatment also were more vigorous and productive than plants from the continuous strawberry treatment during 2002 despite having relatively low foliar biomass and a relatively high level of fungal infection on strawberry plant roots. In the field, symptoms of replant disorder were best overcome by fumigation with methyl bromide or multiple species rotations, particularly that of kale followed by sweet corn and rye. Although Rhizoctonia levels were associated with poor root health, general fungal and bacterial root infection rates were not consistently associated with the presence of visible symptoms of replant disorder nor with strawberry plant growth and productivity.

Specific contributions of decaying alfalfa roots to nitrate leaching in a Kalamazoo loam soil

Abstract Alfalfa ( Medicago sativa L.) contributes 430 million kg N year −1 to the US Corn-Belt soils, according to a 1991 survey. Minimizing leaching losses from these very large N inputs requires a better understanding of the specific root dynamics that relate to the shoot-borne nitrates which have been reported to develop throughout many soil profiles. The objective of the present study was to determine the impact of decaying alfalfa roots on nitrate inputs to soils and on soil hydraulic conductivity properties which affect nitrate leaching. An experiment was initiated in 1994 and data for this report were taken from research on a Kalamazoo loam soil (fine-loamy, mixed, mesic Typic Hapludalf) at the KBS/LTER (long-term ecological research) site in southwestern Michigan, during the period from 1996 through 1997. Soil extractable nitrate (NO 3 -N) and ammonium (NH 4 -N) were monitored to soil depths of 150 cm and soil soluble NO 3 -N and NH 4 -N were monitored by suction lysimeters to the depth of 65 cm. Saturated hydraulic conductivity ( K sat ) of soil was measured by the double-ring infiltrometer method. Following glyphosate termination of the alfalfa stands, nitrate-N released from mineralized alfalfa roots plus shoots totaled 75 kg ha −1 . Alfalfa roots generated 36 kg ha −1 and alfalfa shoots generated 39 kg ha −1 which accumulated in the Ap horizons from April to July in 1997. The presence of decaying alfalfa roots in the profile quadrupled K sat values as compared to bare fallow soils. Nitrates released from decomposing alfalfa roots combined with root-enhanced hydraulic conductivities dramatically increased NO 3 -N leaching following the termination of alfalfa stands. NO 3 -N leaching to deeper horizons approached 83 kg ha −1 in root treatments and 144 kg ha −1 in the root plus shoot treatments during the period from April to December, following alfalfa termination. Our data suggest that under temperate climate such as that of Michigan, groundwater contamination by nitrates can be reduced by terminating alfalfa stands immediately before spring-planting of the subsequent row crop, which can absorb the large quantities of N leaking from decomposing shoots and roots of the legume.

Peroxide oxidation of clay‐associated organic matter in a cultivation chronosequence

SummaryCurrent models of soil organic carbon (SOC) include a passive pool representing refractory soil organic matter (RSOM) with turnover times of hundreds to thousands of years. These models suggest that, as total soil C is depleted, it becomes proportionally enriched in RSOM. The objectives of our study were to quantify clay‐associated organic matter resistant to peroxide treatment in soils presumed to have differing proportions of RSOM, hypothesizing that peroxide‐resistant C in the clay fraction belongs to RSOM, and that its proportion will increase with total C depletion. Clay fractions (&lt; 2 µm) from three soils from a cultivation sequence, differing in the duration of cultivation, one long‐term cultivated soil and one long‐term bare fallow soil corresponded to samples increasingly depleted in total organic C. Samples were suspended in 30% hydrogen peroxide and treated until no changes in C concentration were observed. Total C in the clay‐peroxide suspensions decreased exponentially and displayed kinetics corresponding to labile, intermediate and peroxide‐resistant pools. Carbon isotope analyses showed an enrichment of 13C in samples after peroxide treatment, compared with before, that decreased from 8‰ in forest samples to 0‰ in long‐term bare fallow. The proportion of peroxide‐resistant C did not differ between soils and represented 12% of initial clay‐associated organic C. No proportional increase with soil C depletion was observed and when expressed on a whole‐soil basis, the results underestimated proposed values for the RSOM pool, suggesting that peroxide treatment may not be appropriate for the estimation of the RSOM pool.

Simulating nitrate leaching in bare fallow soils: a model comparison

In Western Europe, agriculture is one of the major contributors to the pollution of ground- and surface waters. Environmental concern has created the need for protection of these waters against eutrophication, caused by nutrient losses from e.g. agricultural sources. Leaching models may be used to predict nitrate-N losses to the environment and a plethora of such leaching models already exist. Four nitrate leaching models (Burns model, SLIM, SACFARM and ANIMO), with varying degree of complexity and parameter requirements, were used to simulate leaching in a bare fallow soil on a number of fields in the Wijlegem catchment in Flanders, Belgium. The models' performance was evaluated both statistically and graphically. Although all models predicted nitrate content in the soil profile within acceptable limits, the slightly adjusted Burns model appeared to simulate both the nitrate nitrogen and the water content best for the calibration field. Similar results were obtained for the evaluation field experiments: the Burns–α model simulated the moisture and nitrate content fairly well, while SLIM performed well in simulating the nitrate content. In conditions with limited data availability, simple (management) models, needing only a limited number of parameters to be measured or calibrated, may yield better simulation results than complex mechanistic models.

Alfalfa Root and Shoot Mulching Effects on Soil Hydraulic Properties and Aggregation

Although alfalfa (Medicago sativa L.) stands have been reported to improve soil physical properties, little is known about the specific influences of above‐ and belowground alfalfa components on soil physical properties. A 2‐yr study was conducted to investigate alfalfa root and shoot mulch modifications of soil physical properties and water movement in the root zone of a Kalamazoo loam soil (fine‐loamy, mixed, mesic Typic Hapludalf) in southwest Michigan. Four treatments were considered: bare fallow (BF), bare fallow with alfalfa shoot mulch (BFSM), alfalfa with shoots removed and roots remaining (AR), and alfalfa with alfalfa shoot mulch (ASM). Volumetric soil water contents were measured by time domain reflectometry (TDR). Development of fine roots was monitored by minirhizotron technology. Alfalfa root systems increased saturated hydraulic conductivity (Ksat) by 57%, total and macroporosities by 1.7 and 1.8%, respectively, and water recharge rate of the soil profile by as much as 5.4% per day. These effects of alfalfa roots on soil porosity were mainly attributed to increased amplitudes of wetting and drying cycles and high rates of root turnover in the Ap horizon. Ksat was significantly correlated with macroporosity Mean weight diameter (MWD) of aggregates from bare fallow soils was 20% higher when alfalfa shoot mulch was applied. Our results suggest that aggregate stability was more affected by C sources from shoot mulch and root turnover than by factors specific to root activities such as physical enmeshment of aggregates and increased soil wetting and drying cycles.

Modifications of Soil Nitrogen Pools in Response to Alfalfa Root Systems and Shoot Mulch

AbstractDecomposing alfalfa (Medicago sativa L.) shoots and roots generate large amounts of NO3‐N available to the next crop but also susceptible to deep leaching. This study was aimed at determining the specific contributions of above‐ and belowground alfalfa biomass to soil N pools. Dynamics of soil and plant N pools were studied in a Kalamazoo loam soil (fine‐loamy, mixed, mesic Typic Hapludalfs) over a 2‐yr period under bare fallow (BF), hare fallow to which alfalfa shoot mulch was applied (BFSM), living alfalfa plants with shoots removed after harvest (A), and living alfalfa with shoot mulch remaining on the soil surface after harvest (ASM). Organic N pools were monitored in alfalfa plant parts, soil‐incorporated debris, and soil organic matter to depths of 150 cm. Inorganic N pools were monitored by suction Iysimeters, soil extraction, and evaluation of soil denitrification rates. Living alfalfa stands kept soil inorganic N at very low levels, whether shoot mulch was applied or not. Soluble inorganic N concentrations decreased earlier in the fall in the upper horizons of bare fallow soils receiving alfalfa shoot mulch, suggesting enhanced leaching from bare soil under alfalfa mulch. Alfalfa crown and roots contained an average of 115 kg N ha−1 after 2 yr of treatment. In conclusion, alfalfa shoot mulch contributed little to sustained increases in soil N pools, while crowns and roots contributed larger quantities to the soil N pool.

A new approach to estimating the pool of stable organic matter in soil using data from long-term field experiments

It is a necessity to have a successful method to separate, quantify and define the active and passive soil organic matter pools for appropriate verification of models. In this study, the organic carbon content of long-term bare fallow soils was used as an indicator of the size of the stable soil organic matter pool. Although soil texture and soil structure are widely accepted as having an influence on the stable pool, most soil organic models neglect the relationship between soil structure and carbon stabilization. Therefore, the aim of this presentation is to estimate the size of the stable carbon pool and to relate it to soil texture and structure properties. It was calculated that over 50 yr, under bare fallow conditions, the relative decrease in the amount of carbon (C) for the most stable pools ranged between 2 and 12%. In comparison, for the less stabilized pools the relative decrease was calculated from 50 to 100%. This indicates that the organic carbon content of long-term bare fallow soils should be very similar to the size of the most stable C pool. We also observed that the amounts of carbon associated with primary particles <20 μm for numerous soils with contrasting carbon content, soil texture, and management practices showed a lower and an upper limit. Both these limits and the carbon content of long-term bare fallow soils (which were assumed to be similar to the size of the stable pool) were related to the content of primary particles <20 μm in the soil. To calculate these relationships, an equation was used including one term to describe the influence of soil texture and another to describe that of soil structure. The calculated regression for the bare fallow soils corresponded very well to the lower limit of carbon content associated with primary particles <20 μm. The upper limit was estimated only by increasing the regression parameter which is related to the amount of C per unit primary particles <20 μm. Considering the many published results of the influence of soil texture and structure on carbon stabilization processes in soil, the stable pool may be defined as the capacity of soils to sorb C. The upper limit of carbon content associated with primary particles <20 μm may be interpreted as the capacity of soil to protect C.

MEASUREMENT OF NITRATE LEACHING POTENTIAL OF A VERTISOL USING BROMIDE AS A TRACER UNDER RAINFED CONDITIONS OF THE INDIAN SEMI-ARID TROPICS

High temperatures and evaporation in summer (April-June) and erratic rainfall in the wet season (July-September) are common features of the harsh environment of Indian semiarid tropics (SAT). The rainfall of this region often occurs in large storms punctuated by discrete dry spells. During excessive rainy periods there is a strong possibility that a large part of the resident soil-N, especially NO − [over] 3, NO − [over] 2 etc., may move beyond the root zone with the excess water percolating down the soil profile. To understand the N losses through leaching in a deep Vertisol (depth > 150 cm) of the Indian SAT, a field experiment was conducted during the "wet" season (June-September) of 1992 under rainfed conditions, using bromide (Br−) as a tracer for NO − [over] 3. Bromide (as NaBr) at 200 kg ha−1 was applied onto the surface of a bare fallow soil, in microplots (200 × 200 cm2), on June 15, and its vertical movement to 100 cm was monitored weekly. Results showed that Br− distribution was influenced strongly by rainfall. After 1 week (June 15-22), with a rainfall of 64 mm, Br− was found to a depth of 60 cm, and whereas most (40%) was in the top layer (0-10 cm), a total of 90% of applied Br− was recovered to a depth of 60 cm. With large storms occurring in early July (e.g., 68 mm on July 2), Br− was moved further down the profile and accumulated largely in the 30 to 40-cm layer. In early August, because of continuous heavy rainfall, almost all Br− had migrated beyond 50 cm depth. Approximately 40% was not found within the profile (0-100 cm) on September 11. In dry spells during late August and early September, upward movement of 10 to 20% of Br− was also noticed at 60 to 100 cm depth. Thus, the results demonstrate that early August may be a critical period for maximum NO − [over] 3 leaching in this soil and emphasize the need of advance strategies to reduce these effects in view of increasing need for N use efficiency and reduction of ground water contamination.

Nitrate leaching from ploughed pasture and the effectiveness of winter catch crops in reducing leaching losses

Abstract The effect of five catch crops (ryecorn, ryegrass, mustard, lupin, bean) on nitrogen (N) leaching following the autumn ploughing of a grass ley was compared with N leaching from bare fallow soil. The concentrations of nitrate N and ammonium N in the drainage water and the quantities of drainage water from the various treatments were measured over a winter period using undisturbed soil monolith lysimeters. Nitrate was the dominant form of N leached. The amount of N leached from soils with the ryegrass catch crop (2.5 kg N/ha) was considerably less than that leached from fallow soil (33 kg N/ha). Nitrate concentrations in the drainage water from soils growing ryecorn and ryegrass were less than 10 mg N/litre, whereas that from lysimeters growing lupins or beans and from the fallow soil exceeded this value. Shoot dry matter yield for mustard (5.91 t/ha) and ryecorn (5.25 t/ ha) was at least twice that of the other crops and very much higher than the 0.49 t/ha produced by the weeds in the fallow trea...

Movement of bromide as a tracer for nitrate in an Alfisol of the Indian Semi-Arid Tropics under rainfed condition

The variable responses of crops to added nitrogen (N) in Alfisols of the Indian semi-arid tropics are partly due to variable rainfall and partly due to variable losses of available-N. To measure the losses of N through leaching, which can be appreciable under some circumstances, a field experiment was conducted during the rainy season (June-September) of 1992, using bromide (Br) as a tracer for NO -3 . Bromide (as NaBr) was applied to bare fallow soil at a rate of 200 kg ha−1 in microplots (2 m × 2 m) and its vertical movement was monitored periodically. Data on rainfall and Br− distribution in the soil profile on different dates of soil sampling clearly indicated that the movement of Br− was strongly dependent on rainfall. During the first month (15 June-15 July) after Br− application, with scattered and light rainfall about 90% of the added Br− remained in the soil profile (0.6 m). After continuous heavy rainfall in early August more than 90% Br− had moved beyond 0.6 m depth. This indicates a very high risk of NO -3 leaching in this soil, and it is unavoidable without special measures to protect the applied N.

Carbon and Nitrogen Distribution in Aggregates from Cultivated and Native Grassland Soils

AbstractLong‐term cultivation of grassland soils reduces soil organic C and N content and has been associated with a deterioration in the aggregate structure of the soil. This study examined the effects of bare fallow (moldboard plow), stubble mulch fallow (subtill), and no‐till fallow management on aggregate size distribution and aggregate organic C and N contents compared with a native (virgin) grassland soil. Aggregate size fractions were separated by wet sieving and the proportion of soil was quantified for each aggregate size class. Mineral‐associated (silt and clay) organic matter was isolated by dispersing aggregates in sodium hexametaphosphate and removing tbe sand and particulate organic matter (POM) by passing the dispersed aggregates through a 53‐µm sieve. The POM fraction is composed primarily of partially decomposed root fragments and has an average C/N ratio of about 16. A large proportion of the total soil dry weight (50–60%) was isolated in the small macroaggregate (250–2000 µm) size class. The native grassland soil was more stable than the cultivated soils when slaked, and the no‐till soil was more stable than the stubble mulch and bare fallow soil when slaked. Reduced tillage management is effective at increasing the proportion of macroaggregates and results in the accumulation of wheat (Triticum aestivum L.) derived POM within the aggregate structure compared with bare fallow soil. It has previously been shown that the POM fraction accounts for the majority of the soil organic matter (SOM) initially lost as a result of cultivation of grassland soils. The data reported in this study relates the loss of structural stability from cultivation to losses of organic C and N from the POM fraction.

Effect of vegetation cover and land slope on runoff and soil losses from the watersheds of Burundi

Soil loss and runoff data were obtained in two consecutive seasons (1986–1987) from 17 runoff-erosion plots which represent different conditions of vegetation cover, slope gradient and slope length. Results revealed that soil loss from bare fallow soil per unit of rainfall and also per unit of runoff increased as the slope gradient increased. The associated runoff-rainfall ratios were always high under the steep slopes. As slope length increased from 5 to 20 m, soil loss per unit of rainfall was doubled and sediment concentration increased, five-fold, while runoff-rainfall ratio decreased to one-half. C u (crop-factor calculated from the Universal Soil Loss Equation (USLE)) values ranged from 0.013 to 0.104, C u, (crop-factor calculated as soil loss ratio from vegetative and fallow plots) from 0.064 to 0.353 and C p from 0.004 to 0.114. The highest values were obtained under cultivated soils and the lowest values under forest and grass soils. Statistical analysis showed that C u or C p (crop-factor related to plant cover characteristics) is recommended in the calculation of the C-factor in the USLE rather than C r which over-estimates the C-factor and thus the total soil loss. Also, cultivation contributed to significant losses of sediment and runoff due to the removal of vegetation cover. The relationship between runoff and soil loss was linear and significant under various conditions of slope and vegetation cover.

Effects of Long-term Fertilization on the Dynamics of Changes of Soil Organic Matter

Summary The soil properties of bare fallow, systematically treated with high doses of fertilizers, changed considerably during the 20 years of the experiment. The organic manuring intensified the mineralization of the carbon, which increased the stability of organic matter. The organic manuring increased the potential ability of soil biocenoses to mineralize and stabilize organic matter, simultansously immobilizing the outer sources of nitrogen. Mineral fertilization did not change the intensity of mineralization of carbon. It decreased the potential activity of microflora and the stability of soil organic matter. The transformations of nitrogen were slow. In the bare fallow soil, fertilized with minerals, the number of microorganisms was strongly reduced. Cellulolytic bacteria were nearly suppressed by mineral fertilization, Azotobacter population completely. The biological component of the soil plays a decisive role in the changes of organic matter in the soil. Organic fertilizers ensure favourable conditions for the microbial activity and a timely start of microbial changes in the soil organic matter. Biochemical-biological methods indicated a decisive improvement of soil fertility after manuring and its worsening after mineral fertilization at extremely high rates for a long period.

Populations of Pseudomonas solanacearum biovar 3 in naturally infested soil

Populations of Pseudomonas solanacearum biovar 3 were monitored in a clay loam soil sampled from the root zone of infected tomato plants during 1978, 1979 and 1980. Soil numbers increased during symptom development and declined with the death of infected plants. The decline in population size in the soil was continuous where no cover crop was planted between the autumn and spring crops. This decline in population size was interrupted, however, following the planting of an oats cover crop numbers decreased with the ploughing under of the oats. Rainfall was associated with high soil numbers but soil temperature did not appear to directly affect population size. Soil populations in the root zone of susceptible tomato plants cultivar Floradel reached a maximum 1000-fold greater than in soil from the root zone of a resistant line. P. solanacearum survived in bare fallow soil for 21 months. Tomatoes planted 2 months later wilted rapidly.

Variation in Larval Population and Development of Meloidogyne Naasi in Field Soil

Second stage larvae of Meloidogyne naasi could be extracted from soil at all times throughout the year but in much greater numbers in spring. Incubation of soil at 20° for a week prior to extraction, or storage at 10° for 4 weeks before such incubation, increased the number of larvae extracted especially between August and April. Nematodes disappeared from soil beneath crops faster than from bare-fallow soil. Only one complete generation of M, naasi was possible in a year.