Experimental assessment of runoff and soil erosion in an olive grove on a Vertic soil in southern Spain as affected by soil management

Abstract. Three different management systems were compared in an olive grove on a Vertic soil, near the city of Cordoba, Spain. Rainfall, runoff and soil loss were recorded from experimental plots of 6×12 m for three years. Results indicated that the no-tillage system, which was kept weed-free with herbicides, gave the largest soil loss (8.5 t ha⁻¹ yr⁻¹) and average annual runoff coefficient (21.5%), due to increased soil compaction, particularly outside the canopy projection area. A system that used a grass cover gave the lowest soil losses (1.2 t ha⁻¹ yr⁻¹) and average annual runoff coefficient (2.5%) due to the protective effects of the cover and increased soil aggregate stability. The third system, conventional tillage, gave intermediate results, with a soil loss of 4.0 t ha⁻¹ yr⁻¹ and an average runoff coefficient of 7.4%. The search for alternative soil management to conventional tillage should consider occasional light tillage to establish a grass cover that would keep both soil erosion and runoff losses to a minimum.     

Fine-earth translocation by tillage in stony soils in the Guadalentin, south-east Spain: an investigation using caesium-134

Tillage erosion is increasingly recognised as an important soil erosion process on agricultural land. In view of its potential significance, there is a clear need to broaden the experimental database for the magnitude of tillage erosion to include a range of tillage implements and agricultural environments. The study discussed in this paper sought to address the need for such data by examining tillage erosion by a duckfoot chisel plough in stony soils on steep slopes in a semi-arid environment. Results of the investigation of coarse fraction (rock fragment) translocation by tillage in this environment have been presented elsewhere and the paper focuses on tillage translocation and erosion of the fine earth. Tillage translocation was measured at 10 sites, representing both upslope and downslope tillage by a duckfoot chisel plough on five different slopes, with tangents ranging from 0.02 to 0.41. A fine-earth tracer, comprising fine earth labelled with ¹³⁴Cs, was introduced into the plough layer before tillage. After a single pass of the plough, incremental samples of plough soil were excavated and sieved to separate the fine earth from the rock fragments. Translocation of the fine-earth tracer was established by analysing the ¹³⁴Cs content of the samples of fine earth. These data were used to establish translocation distances for each combination of slope and tillage direction. Translocation distances of the fine earth were not significantly different from translocation distances of the coarse fraction. For all sites, except uphill on the 0.41 slope, translocation distances were found to be linearly related to slope tangent. The soil flux due to tillage for each site was calculated using the translocation distance and the mass per unit area of the plough layer. For slopes with tangents <0.25, the relationship between soil flux and tangent was linear and the soil flux coefficient derived was 520–660 kg m⁻¹ per pass. This is much larger than the coefficients found in other studies and this high magnitude is attributed to the non-cohesive nature and high rock fragment content of the soil in this investigation. A second contrast with previous studies was found in non-linearity in the relationship between soil flux and tangent when steeper slopes were included. This was a product of variation in plough depth between the steepest slopes and the remainder of the study area. On the basis of the study it is suggested that an improved understanding of tillage erosion may be obtained by considering the dual processes of tillage detachment (mass per unit area of soil subject to tillage) and tillage displacement (equivalent to translocation distance per pass) in assessing, comparing and modelling tillage translocation. An improved model is proposed that recognises the complexity of soil redistribution by tillage, provides a framework for process-based investigation of the controls on tillage fluxes, and allows identification of potential self-limiting conditions for tillage erosion.     

Subsidence rates and carbon loss in peat soils following conversion to pasture in the Waikato Region, New Zealand

Abstract. Drainage of peat soils for agriculture can lead to large carbon losses due to oxidation of peat. We estimated peat subsidence rates and total carbon losses, due to 40 years of dairy farming on a former peat bog, by measuring the thickness of peat and total carbon of farmland and of an adjacent unmodified peat bog above a marker tephra layer that was deposited about 200 AD. Subsidence rates averaged 3.4 cm yr^–1 (95% confidence interval of 3.2 to 3.5 cm yr^–1) and carbon loss averaged 3.7 t ha^–1 yr^–1 (95% confidence interval of 2.5 to 5.0 t ha^–1 yr^–1). On average, 63% of the subsidence was due to consolidation, with the remainder (37%) attributed to losses of organic matter due to peat mineralization.     

Modelling tillage erosion in the topographically complex landscapes of southwestern Ontario, Canada

A tillage erosion model was developed for southwestern Ontario based on the relationship between tillage translocation and slope gradient and slope curvature. Two studies of tillage translocation and tillage erosion were used to calibrate this model, one a comparison of upslope and downslope tillage translocation on shoulder slopes, the other an examination of tillage translocation throughout topographically complex landscapes. Two field sites were used for validation of the model. For both sites, past tillage practices were known and past soil erosion was determined using ¹³⁷Cs as an indicator of soil redistribution. The model accurately predicted the pattern of soil redistribution that had occurred within the two field sites. Severe soil loss was observed and predicted on convex landscape positions and soil accumulation was observed and predicted on concave landscape positions. The model accounted for almost all of the soil lost from the convex upper slope positions where tillage erosion was expected to be the dominant erosion process. There was considerable soil loss and accumulation elsewhere in the landscapes which could not be accounted for by the model and was presumed to be primarily the result of water erosion. It was concluded that tillage erosion must be incorporated into soil erosion modelling for the purposes of soil conservation.     

Effectiveness of stone bunds in controlling soil erosion on cropland in the Tigray Highlands, northern Ethiopia

Abstract Use of stone bunds to enhance soil and water conservation was first introduced to Tigray, northern Ethiopia in 1981. This study was designed to examine the factors that control the effectiveness of bunds installed on cropland. Qualitative and quantitative assessments of soil loss and sediment accumulation were conducted on 202 plots at 12 representative sites in Dogu'a Tembien district. Mean annual soil loss from the foot of the bunds due to tillage erosion was estimated at 39kgm⁻¹yr⁻¹ or 20tha⁻¹yr⁻¹, a rate which decreased with increasing age of bund. The assessed mean annual soil loss rate by sheet and rill erosion in the absence of stone bunds is 57tha⁻¹yr⁻¹. The mean measured annual rate of sediment accumulation behind the stone bunds is 119kgm⁻¹yr^−1/sp or 59tha⁻¹yr⁻¹. The measurements show that the introduction of stone bunds to the region has led to a 68% reduction in annual soil loss due to water erosion. This reduction is due to the accumulation of sediment behind the stone bunds, which occurs faster in the early years after construction and decreases as the depression behind the bunds becomes filled with sediment. New stone bunds are particularly effective in trapping sediment in transport, but regular maintenance and increase in height of the bunds is necessary to maintain their effectiveness. The average USLE P factor for stone bunds in the study area is estimated to be 0.32.     

Measurement and modelling of the effects of initial soil conditions and slope gradient on soil translocation by tillage

Tillage erosion studies have mainly focused on the effect of topography and cultivation practices on soil translocation during tillage. However, the possible effect of initial soil conditions on soil displacement and soil erosion during tillage have not been considered. This study aims at investigating the effect of the initial soil conditions on net soil displacement and the associated erosion rates by a given tillage operation of a stony loam soil. Tillage erosion experiments were carried out with a mouldboard plough on a freshly ploughed (pre-tilled) soil and a soil under grass fallow in the Alentejo region (Southern Portugal).

The experimental results show that both the downslope displacement of soil material and the rate of increase of the downslope displacement with slope gradient are greater when the soil is initially in a loose condition. This was attributed to: (i) a greater tillage depth on the pre-tilled soil and (ii) a reduced internal cohesion of the pre-tilled soil, allowing clods to roll and/or slide down the plough furrow after being overturned by the mouldboard plough.

An analysis of additional available data on soil translocation by mouldboard tillage showed that downslope displacement distances were only significantly related to the slope gradient when tillage is carried out in the downslope direction. When tillage is carried out in the upslope direction, the effect of slope gradient on upslope displacement distances was not significant. This has important implications for the estimation of the tillage transport coefficient, which is a measure for the intensity of tillage erosion, from experimental data. For our experiments, estimated values of the tillage transport coefficient were 70 and 254 kg m⁻¹ per tillage operation for grass fallow and pre-tilled conditions, respectively, corresponding to local maximum erosion rates of ca. 8 and 35 Mg ha⁻¹ per tillage operation and local maximum deposition rates of ca. 33 and 109 Mg ha⁻¹ per tillage operation.     

Soil translocation by weeding on steep-slope swidden fields in northern Vietnam

This study investigated soil translocation associated with weeding on steeply sloped swidden fields attended by ethnic Da Bac Tay farmers in Hoa Binh Province in northern Vietnam. Annual soil loss rates of 4–6 Mg ha⁻¹ year⁻¹ were found on 20 m plots located on two separate hillslopes. Median soil flux rates were equivalent to 2.6–3.9 kg m⁻¹ pass⁻¹ for experiments conducted on slopes ranging from 0.54 to 0.84 m m⁻¹. The primary soil translocation process, the mechanical movement of soil via contact with a small hoe (ngheo), contributed approximately 60% of the weeding-related soil flux. Ravel, which is the rolling, bouncing, and sliding of soil clods downslope, was a secondary translocation process that accounted for almost 40% of the soil flux. Soil flux was most appropriately described with an exponential function that could predict the occurrence of ravel on steeper slopes. The observed soil fluxes were much smaller than those determined during weeding and hoeing at other tropical and subtropical sites, primarily because the tillage depth was very shallow (<1 cm) and weed density was low at the time of experimentation. The erosion rates associated with weeding were an order of magnitude lower than reported water erosion rates; therefore, the contribution to landscape change was believed to be small. Combined water and tillage erosion estimates indicated a possible unsustainable increase in soil loss on some steep-slope fields within the last few decades that has resulted from shorter fallow periods, longer periods of cultivation before fallowing, and greater weed pressure. Additional work is needed to verify these latter interpretations.     

Tillage-induced CO2 loss across an eroded landscape

Soil carbon (C) losses and soil translocation from tillage operations have been identified as causes of soil degradation and soil erosion. The objective of this work was to quantify the variability in tillage-induced carbon dioxide (CO₂) loss by moldboard (MP) and chisel (CP) plowing across an eroded landscape and relate the C loss to soil properties. The study site was a 4 ha wheat (Triticum aestivum L. cv. Marshall) field with rolling topography and five soil types in the Svea-Barnes complex in west central Minnesota (N. Latitude = 45°41′W, Longitude = 95°43′). Soil properties were measured at several depths at a 10 m spacing along north–south (N–S) and west–east (W–E) transects through severely eroded, moderately eroded and non-eroded sites. Conventional MP (25 cm deep) and CP (15 cm deep) equipment were used along the pre-marked transects. Gas exchange measurements were obtained with a large, portable chamber within 2 m of each sample site following tillage. The measured CO₂ fluxes were largest with the MP > CP > not tilled (before tillage). The variation in 24 h cumulative CO₂ flux from MP was nearly 3-fold on the N–S transect and 4-fold on the W–E transect. The surface soil organic C on the transects was lowest on the eroded knolls at 5.1 g C kg⁻¹ and increased to 19.6 g C kg⁻¹ in the depositional areas. The lowest CO₂ fluxes were measured from severely eroded sites which indicated that the variation in CO₂ loss was partially reflected by the degradation of soil properties caused by historic tillage-induced soil translocation with some wind and water erosion.

The spatial variation across the rolling landscape complicates the determination of non-point sources of soil C loss and suggests the need for improved conservation tillage methods to maintain soil and air quality in agricultural production systems.     

The effects of stubble burning and tillage on soil carbon sequestration and crop productivity in southeastern Australia

Abstract. In Australia, stubble burning and tillage are two of the major processes responsible for the decline of soil organic carbon concentration in cropped soils, and the resulting soil degradation. However, the relative importance of these two practices in influencing the soil organic carbon concentration and the long-term impact on soil quality and productivity are not clear. The effects of stubble burning as practised by farmers in southeastern Australia were evaluated in two field trials, one of 19 years duration, the other of 5 years. Conventional tillage (three tillage passes) led to greater loss of soil organic carbon than stubble burning. Loss of total soil organic carbon attributed to stubble burning in the 0–10 cm layer was estimated to be 1.75 t C ha⁻¹ over the period of the 19-year trial, equivalent to 29% of that lost due to tillage. In the 5-year trial, no change in soil organic carbon due to stubble burning was detectable. Changes in soil quality associated with stubble burning detected in the longer trial included a reduction in macro-aggregate stability, and increases in pH and exchangeable K⁺. Only the latter two were detected in the shorter trial. A higher mean wheat yield (average 0.15 t ha⁻¹) following stubble burning was observed in the 19-year trial but not in the 5-year trial. Research to monitor the longer term effects of stubble burning is needed, and to identify conditions where loss of soil organic carbon is minimized.     

The effects of minimal tillage and contour cultivation on surface runoff, soil loss and crop yield in the long-term Woburn Erosion Reference Experiment on sandy soil at Woburn, England

Abstract. Over a 10-year period, runoff and soil erosion on the plots of the Woburn Erosion Reference Experiment were concentrated in periods with sparse vegetation cover: in winter after the late planting of cereals; in spring after the planting of beets; or when soils were bare after harvest. The mean event runoff of 1.32 mm from plots cultivated up-and-downslope was significantly greater ( P <0.05) than that from plots cultivated across-slope (0.82 mm). However, mean event soil loss was not significantly different between the two cultivation directions. No significant differences were found between minimal and standard cultivations. Mean event runoff from the across-slope/minimal tillage treatment combination (0.58 mm) was significantly less ( P <0.01) than from the up-and-downslope/minimal tillage (1.41 mm), up-and-downslope/standard tillage (1.24 mm), and across-slope/standard tillage (1.07 mm) treatment combinations. Runoff from the across-slope/standard treatment combination was significantly ( P <0.05) less than from the up-and-downslope/minimal tillage treatment. The across-slope/minimal tillage treatment combination had a significantly smaller ( P <0.05) event soil loss (67 kg ha⁻¹) than the up-and-downslope/standard tillage (278 kg ha⁻¹) and up-and-downslope/minimal tillage (245 kg ha⁻¹) combinations. Crop yields were significantly ( P <0.05) higher on across-slope plots in 1988, 1996 and 1997 than on up-and-downslope plots, and were also higher (but not significantly) on the across-slope plots in 7 of the 8 remaining years. Minimal cultivation decreased yield compared with standard cultivation in one year only. We recommend that across-slope cultivation combined with minimal tillage be investigated at field scale to assess its suitability for incorporation into UK farming systems.     

Assessment of tillage erosion rates on steepland Oxisols in the humid tropics using granite rocks

Soil translocation by tillage may be an important factor in land degradation in the humid tropics. The objective of this study was to evaluate tillage-induced soil translocation on an Oxisol with 25% and 36% slopes in Claveria, Philippines for three tillage systems: contour moldboard plowing (CMP), moldboard plowing up and downslope (UMP), and contour ridge tillage (CRT). Small rocks 3–4 cm in “diameter” were used as soil movement detection units (SMDU). The SMDUs were placed at 10 cm intervals in a narrow 5-cm-deep trench near the upper boundary of each plot, the position of each rock recorded, and the trench backfilled. Five tillage operations used to produce one corn crop were performed during a one month period: two moldboard plowing operations for land preparation (except for CRT), one moldboard plowing for corn planting, and two inter-culture (inter-row cultivation) operations. After these operations, over 95% of the SMDU were recovered manually and their exact locations recorded. Mean annual soil flux for the 25% slope was 365 and 306 kg m⁻¹ y⁻¹ for UMP and CMP, respectively. For the 36% slope, comparable values were 481 and 478 kg m⁻¹ y⁻¹. Estimated tillage erosion rates for the 25% slope were 456 and 382 Mg ha⁻¹ y⁻¹ for UMP and CMP, respectively, and increased to 601 and 598 Mg ha⁻¹ y⁻¹, respectively, for the 36% slope. The mean displacement distance, mean annual soil flux, and mean annual tillage-induced soil loss for both slopes were reduced by approximately 70% using CRT compared to CMP and UMP.     

Implement and soil condition effects on tillage-induced erosion

Water, wind, or tillage-induced soil erosion can significantly degrade soil quality. Therefore, understanding soil displacement through tillage translocation is an important step toward developing tillage practices that do not degrade soil resources. Our primary objective was to determine the effects of soil condition (i.e. grassland stubble versus previously tilled soil), opening angle, and harrow speed on soil translocation. A second field study also conducted on a Lixisol but only in the stubble field, quantified displacement effects of mouldboard ploughing. The field studies were located 12 km South of Évora, Portugal. Soil displacement or translocation after each tillage operation in both studies was measured using aluminium cubes with a side length of 15 mm as ‘tracers’. Offset angles for the harrow disk were 20°, 44° and 59°; tractor velocities ranged from 1.9 to 7.0 km h⁻¹ and tillage depth ranged from 4 to 11 cm. The depth of mouldboard ploughing was approximately 40 cm with a wheel speed of 3.7 km h⁻¹. The translocation coefficients for the two implements were very different averaging 770 kg m⁻¹ for the mouldboard plough and ranging from 9 to 333 kg m⁻¹ for the harrow disk. This shows that the mouldboard plough was more erosive than the harrow disk in these studies. All three variables (soil condition, opening angle, and tillage velocity) were critical factors affecting the translocation coefficient for the harrow disk. Displacement distances were the largest for compacted soils (stubble field), with higher opening or offset angles, and at higher velocities. The results also showed significant correlation for (a) mean soil displacement in the direction of tillage and the slope gradient and (b) soil transport coefficient and the opening angle. Our results can be used to predict the transport coefficient (a potential soil quality indicator for tillage erosion) for the harrow disk, provided tillage depth, opening angle, and tool operating speed are known.     

Effect of tillage practices on wheat performance in a semi-arid environment

Sodosol soils are at risk of degradation under existing fallow management practices involving tillage. Topsoil erosion exposes horizons with reduced infiltration and low concentrations of plant nutrients. Conservation management systems are needed on these soils to avoid a reversion to low intensity grazing. This paper reports on a 4 year study (1986–1989) of the effects of tillage practices on profile soil water and crop yield in a Sodosol (Typic Natrustalf) in central Queensland, Australia. The tillage treatments were: zero till fallow (weed control by herbicides), reduced till fallow (chisel plough/scarifier or herbicides) and conventional till fallow (chisel plough/scarifier) in two linked experiments. In the first experiment, wheat was grown in three contour bays (approximately 1 ha), and in the second, wheat was grown in replicated plots (30 m × 6 m) to allow statistical comparisons.

Zero till provided consistent advantages in grain yield in all 4 years compared with conventional till. Zero till also outyielded reduced till as well as conventional till in the plot experiment. The average yield increase of 0.5 t ha⁻¹ in zero till compared with convention till was associated with greater water use and increased water use efficiency. Tillage practice caused only marginal differences in the available water content in the root zone (0–100 cm) at sowing; zero and reduced till contained, on average, an additional 4 and 8 mm, respectively, compared with conventional till. The tillage treatments had no effect on plant available water capacity. Some of the soil water that accumulated during the fallow drained beyond the root zone in all treatments and was not available to the following wheat crop. At the conclusion of the experiment, soil water accumulation in the 100–180 cm soil layer was 86 mm in zero till, 39 mm in reduced till and 40 mm in conventional till.

Results indicate that zero till can be a more productive wheat farming practice than conventional mechanical tillage. The increase in water storage below the root zone of the wheat crop shows that there may be benefit in using a deeper-rooting crop or pasture species in rotation with wheat, particularly after zero till fallows.     

Characterization of soil profiles in a landscape affected by long-term tillage

Soil movement by tillage redistributes soil within the profile and throughout the landscape, resulting in soil removal from convex slope positions and soil accumulation in concave slope positions. Previous investigations of the spatial variability in surface soil properties and crop yield in a glacial till landscape in west central Minnesota indicated that wheat (Triticum aestivum) yields were decreased in upper hillslope positions affected by high soil erosion loss. In the present study, soil cores were collected and characterized to indicate the effects of long-term intensive tillage on soil properties as a function of depth and tillage erosion. This study provides quantitative measures of the chemical and physical properties of soil profiles in a landscape subject to prolonged tillage erosion, and compares the properties of soil profiles in areas of differing rates of tillage erosion and an uncultivated hillslope. These comparisons emphasize the influence of soil translocation within the landscape by tillage on soil profile characteristics. Soil profiles in areas subject to soil loss by tillage erosion >20 Mg ha⁻¹ year⁻¹ were characterized by truncated profiles, a shallow depth to the C horizon (mean upper boundary 75 cm from the soil surface), a calcic subsoil and a tilled layer containing 19 g kg⁻¹ of inorganic carbon. In contrast, profiles in areas of soil accumulation by tillage >10 Mg ha⁻¹ year⁻¹ exhibited thick sola with low inorganic carbon content (mean 3 g kg⁻¹) and a large depth to the C horizon (usually >1.5 m below the soil surface). When compared to areas of soil accumulation, organic carbon, total nitrogen and Olsen-extractable phosphorus contents measured lower, whereas inorganic carbon content, pH and soil strength measured higher throughout the profile in eroded landscape positions because of the reduced soil organic matter content and the influence of calcic subsoil material. The mean surface soil organic carbon and total nitrogen contents in cultivated areas (regardless of erosion status) were less than half that measured in an uncultivated area, indicating that intensive tillage and cropping has significantly depleted the surface soil organic matter in this landscape. Prolonged intensive tillage and cropping at this site has effectively removed at least 20 cm of soil from the upper hillslope positions.     

Effect of plot scale and an upslope phosphorus source on phosphorus loss in overland flow

Abstract. Phosphorus (P) in overland flow is mediated by soil P, added P, erosion, and hydrological processes and their interaction as affected by landscape position and length of flow. We investigated the effect of flow path length (1 to 10 m long plots) on P transport in overland flow with and without a localized dairy manure application (75 kg P ha^–1 added to the upslope end [0.5 m] of each plot) and simulated rainfall (7 cm h^–1), at two sites within an agricultural watershed in Pennsylvania, USA. Particulate loss in overland flow was c . 20% greater from manured than unmanured plots due to the less dense nature of manure than soil. Increased soil moisture at Site 2 contributed to a greater loss of P compared to Site 1, both with and without manure; with most occurring as particulate P (60 to 90% of total P). Further, the selective erosion of fine particulates (24 to 34% clay) and P loss increased with plot length. From a management perspective our results demonstrate that the forms and amounts of P loss are greatly influenced by flow path length and interactions among antecedent moisture, soil P, and texture.     

The effects of cover crops and conventional tillage on soil and runoff loss in vineyards and olive groves in several Mediterranean countries

Cover crops (CC) in vineyards and olive groves provide an alternative to conventional tillage (CT) for land management. Runoff, sediment and nutrient loss from six sites in France, Spain and Portugal were compared over 3–4 yr. In general, runoff loss was not significantly reduced by the CC alternatives: average annual runoff coefficients ranged from 4.9 to 22.8% in CT compared with 1.9–25% in the CC alternatives. However, at two sites, reductions in average annual runoff coefficients were greater for CC: 17.2 and 10.4% in CT, 6.1 and 1.9% in CC. Nutrient loss in runoff followed a similar pattern to runoff, as did pesticide loss on the one site; reductions occurred when runoff losses were significantly reduced by CC. The lack of differences at the other sites is thought to be due to a combination of soil conditions at the surface (compaction and capping) and sub‐surface (low‐permeability horizons close to the surface). In contrast, CC always resulted in reductions in soil erosion loss, plus similar reductions in nutrients and organic matter (OM) associated with sediment. Soil erosion loss ranged from 1.4 to 90 t/ha/yr in CT compared with 0.04–42.7 t/ha/yr in CC. Overall, reductions in runoff and associated nutrient and pesticide loss from vineyards and olives occurred with the introduction of CCs only when soil permeability was sufficiently high to reduce runoff. In contrast, reduction in soil erosion and associated nutrients and OM occurred even when the amount of runoff was not reduced. In the most extreme encountered situations (highly erodible soils in vulnerable landscape positions and subject to highly erosive rainfall), additional conservation measures are needed to prevent unsustainable soil loss.     

Soil management for Alfisols in the semiarid tropics: erosion, enrichment ratios and runoff

Abstract. Continuous cultivation of soils of the semiarid tropics has led to significant land degradation. Soil erosion and nutrient loss caused by high runoff volumes have reduced crop yields and contributed to offsite damage. We compared a number of soil management practices (tillage, mulch and perennial/annual rotational based systems) for their potential to improve crop production and land resource protection in an Alfisol of the semiarid tropics of India. Runoff and soil erosion were monitored and surface soil and sediment were analysed for nitrogen and carbon to determine enrichment ratios. Amelioration of soils with organic additions (farmyard manure, rice straw) or rotating perennial pasture with annual crops increased soil carbon and nitrogen contents and reduced runoff, soil erosion and nutrient loss. Soil erosion totalled less than 7 t ha^–1, but enrichment ratios were often greater than 2 resulting in up to 27 kg N ha^–1 and 178 kg C ha^–1 being lost in sediment. Up to an extra 250 mm of water per year infiltrated the soil with organic additions and was available for crop water use or percolation to groundwater. The results show that there are good opportunities for reducing degradation and increasing productivity on farms.     

A review of farm-scale nutrient budgets for organic farms as a tool for management of soil fertility

Abstract. On organic farms, where the importation of materials to build/maintain soil fertility is restricted, it is important that a balance between inputs and outputs of nutrients is achieved to ensure both short-term productivity and long-term sustainability. This paper considers different approaches to nutrient budgeting on organic farms and evaluates the sources of bias in the measurements and/or estimates of the nutrient inputs and outputs. The paper collates 88 nutrient budgets compiled at the farm scale in nine temperate countries. All the nitrogen (N) budgets showed an N surplus (average 83.2 kg N ha^–1 yr^–1). The efficiency of N use, defined as outputs/inputs, was highest (0.9) and lowest (0.2) in arable and beef systems respectively. The phosphorus (P) and potassium (K) budgets showed both surpluses and deficits (average 3.6 kg P ha^–1 yr^–1, 14.2 kg K ha^–1 yr^–1) with horticultural systems showing large surpluses resulting from purchased manure. The estimation of N fixation and quantities of nutrients in purchased manures may introduce significant errors in nutrient budgets. Overall, the data illustrate the diversity of management systems in place on organic farms, and suggest that used together with soil analysis, nutrient budgets are a useful tool for improving the long-term sustainability of organic systems.     

Effect of no-tillage on turnover of organic matter in a Rhodic Ferralsol

Abstract. Soil organic matter (SOM) is considered to be key to sustainability of agriculture in the tropics. In southern Brazil, no-tillage has been adopted widely to control soil erosion, but its impact on the dynamics of SOM is not well established. We measured soil carbon (C) and δ¹³C in two crop rotations, one of which contained C4 maize ( Zea mays L.), after 21 years of contrasting tillage (conventional tillage versus no-tillage). Adjacent sites that reflected historic land-uses were also sampled. In the tillage experiment there was no effect of tillage on the total amount of C in the 0–40 cm profile (even when contrasting bulk density was accounted for), and the concentration of C differed only in the 0–5 cm and 5–10 cm layers. However, the occasional input of C4 material in the maize rotation resulted in a significant effect of rotation on δ¹³C ( P <0.001). Using ¹³C as a tracer for the SOM formed since the start of the experiment, we estimated the abundance of 'recent' and 'old' C within each depth interval. We found the main effect of tillage was to increase the medium-term turnover of SOM, particularly in the subsoil (i.e. below 20 cm depth). Compared with no-tillage, there was almost five times more recent C in the subsoil, and 20% more recent C in the 0–40 cm profile as a whole.     

保护性耕作对农田地表径流与土壤水蚀影响的试验研究

在黄土坡地建立天然降雨径流小区,采用翻斗式自动测试系统同步动态地监测降雨—径流的过程,试验研究了保护性耕作农田水土保持的效果和耕作、覆盖及压实3种因素对农田水土流失的影响。2年的试验表明,雨强和雨型与坡地水土流失密切相关,在暴雨情况下,由秸秆覆盖与少免耕相结合的保护性耕作具有明显的保持水土作用;采用少免耕而无秸秆覆盖配合的情况下,水土流失甚至高于传统翻耕。在试验的6种处理中,免耕覆盖不压实的保水保土效果最佳,相对传统翻耕年径流量减少52.5%,年土壤流失量减少80.2%。在覆盖、压实及耕作3因素中,秸秆覆盖对保持水土的作用最大,可减少年径流量47.3%,减少年土壤水蚀77.6%;压实次之,地表耕作的影响较小。