Fallow System Research Articles

Coppicing improved fallows are profitable for maize production in striga infested soils of western Kenya

Strigahermonthica (striga) weed is a major threat to crop production in sub-Saharan Africa, and short duration improved fallow species have recently been found to reduce the effects of this weed because of their ability to replenish soil nitrogen. The objective of this study was to compare the efficacy and profitability of coppicing improved fallow species (Gliricidia sepium [gliricidia], Leucaena trichandra [leucaena] and Calliandra calothyrsus [calliandra]) and non-coppicing species (Sesbania sesban [sesbania], Mucuna pruriens [mucuna], and Tephrosia vogelii [tephrosia]), in controlling striga. Natural fallow and a sole maize crop were included as control treatments. The fallow treatments were split into two and either fertilized with N or unfertilized. The results showed that coppicing fallows produced higher biomass than non-coppicing fallows. For example, Callindra (coppicing fallow species) produced 19.5 and 41.4 Mg ha−1 of leafy and woody biomass, respectively after four cumulative harvests as compared with Sesbania (non-coppicing species), which produced only 2.3 and 5.9 Mg ha−1 leaf and woody biomass, respectively. Improved fallows reduced striga population in proportion to the amount of leafy biomass incorporated into the soil (r = 0.87). N application increased cumulative maize yield by between 15–28% in improved fallow systems and by as much as 51–83% in the control treatments. Added total costs of the coppicing fallows did not differ significantly from those of the non-coppicing fallows and control treatments. However, the added net benefits of the coppicing fallows were significantly higher (US$ 527 for +N and 428 for −N subplots; P < 0.01) than those of the non-coppicing fallows (US$ 374 for +N and 278 for −N), and the least for the control treatments. The most profitable fallow system was Tephrosia with net added benefits of US$ 453.5 ha−1 season−1 without N, and US$ 586.7 ha−1 season−1 with added N.

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

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

Modeling N2O flux from an Illinois agroecosystem using Monte Carlo sampling of field observations

We modeled the expected range of seasonal and annual N2O flux from temperate, grain agroecosystems using Monte Carlo sampling of N2O flux field observations. This analysis is complimentary to mechanistic biogeochemical model outcomes and provides an alternative method of estimating N2O flux. Our analysis produced a range of annual N2O gas flux estimates with mean values overlapping with results from an intermodel comparison of mechanistic models. Mean seasonal N2O flux was 1–4% of available N, while median seasonal N2O flux was less than 2% of available N across corn, soybean, wheat, ryegrass, legume, and bare fallow systems. The 25th–75th percentile values for simulated average annualized N2O flux rates ranged from 1 to 12.2 kg N ha−1 in the conventional system, from 1.3 to 8.8 kg N ha−1 in the cover crop rotation, and from 0.8 to 9.3 kg N ha−1 in the legume rotation. Although these modeling techniques lack the seasonal resolution of mechanistic models, model outcomes are based on measured field observations. Given the large variation in seasonal N gas flux predictions resulting from the application of mechanistic simulation models, this data-derived approach is a complimentary benchmark for assessing the impact of agricultural policy on greenhouse gas emissions.

Earthworm populations in relation to soil organic matter dynamics and management in California tomato cropping systems

Earthworms are key regulators of soil structure and soil organic matter (SOM) dynamics in many agroecosystems. They are greatly impacted by agricultural management, yet little is known about how these factors interact to control SOM dynamics. This study sought to explore linkages between agricultural management, earthworms and aggregate associated SOM dynamics through a survey of tomato ( Solanum lycopersicum L.) cropping systems in northern California. Earthworms and soil samples were collected between February and April of 2005 from 16 fields under one of three types of residue management: (1) tomato mulch – no postharvest tillage and tomato residues left on the soil surface, (2) cover crop – tomato residues tilled in and leguminous cover crop planted, and (3) bare fallow – tomato residues tilled in and soil surface left exposed throughout the winter. Earthworms were collected via hand-sorting and identified to species, while soils were wet sieved to yield four aggregate size classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm) and the silt and clay fraction (<53 μm). The combined large and small macroaggregate fraction was then fractionated into coarse particulate organic matter (cPOM; 250 μm), microaggregates within macroaggregates (mM; 53–250 μm) and macroaggregate occluded silt and clay (Msc; <53 μm). The earthworms identified in this survey were composed entirely of exotic species and were dominated by Aporrectodea caliginosa. Earthworm abundance was related to residue management, with the tomato mulch systems averaging 4.5 times greater fresh earthworm biomass than bare fallow ( P = 0.024). Aggregate stability and total soil C and N also appeared to be influenced by residue management, such that the tomato mulch system displayed significantly greater mean weight diameters than the bare fallow system ( P = 0.049), as well as more than 50% greater total soil C and N ( P = 0.049 and P = 0.036; respectively). Earthworm biomass was also found to be positively correlated with total soil C ( P = 0.009, R 2 = 0.39) and N ( P = 0.010, R 2 = 0.039) as well as the proportion of macroaggregate C in the cPOM fraction ( P = 0.028, R 2 = 0.30). Our findings suggest that residue handling and the associated management practices (e.g., tillage, organic vs. conventional agriculture) are important for both earthworm populations and SOM storage. Although earthworms are known to influence SOM in many ways, other factors appear to play a more prominent role in governing aggregate associated SOM dynamics.

Jointed Goatgrass Management with Imazamox‐Resistant Cultivars in a Winter Wheat–Fallow Rotation

Jointed goatgrass (Aegilops cylindrica Host) is an economically important weed in winter wheat (Triticum aestivum L.)–fallow production regions of the United States. Few options exist for management of jointed goatgrass in winter wheat due to the close genetic relationship between the two species. This research evaluated use of imazamox herbicide (2‐[(RS)‐4‐isopropyl‐4‐methyl‐5‐oxo‐2‐imidazolin‐2‐yl]‐5‐methoxymethylnicotinic acid) in an imazamox‐resistant (IR) winter wheat–fallow rotation over 6 yr when grown continuously or in rotation with a conventional winter wheat cultivar. The objectives of this study were to compare use patterns of IR technology in winter wheat with respect to jointed goatgrass density and wheat grain contamination over three crop years in a winter wheat–fallow system and to monitor jointed goatgrass and jointed goatgrass × wheat hybrid populations for resistance to imazamox. The IR treatment resulted in reduced jointed goatgrass density compared to conventional treatments in 5 out of 6 yr. Reductions in jointed goatgrass tiller density resulting from use of imazamox in IR winter wheat were evident in a subsequent crop year at each of two experimental sites. No IR jointed goatgrass plants nor viable jointed goatgrass × wheat hybrid plants were found. In a winter wheat–fallow rotation, IR technology can effectively manage jointed goatgrass populations and reduce jointed goatgrass contamination of grain. Use of the technology every other crop year, or two out of every three crop years, will provide management benefits in subsequent years.

Soil microbial biomass and mineralization of aggregate protected carbon in fallow-maize systems under conventional and no-tillage in Central Zimbabwe

Abstract Soil organic carbon has been reported to increase soil microbial activities in soils. However, very few studies have evaluated soil microbial biomass carbon (SMBC), aggregate protected carbon, and mineralizable carbon, SOC associated with sand, silt and clay and aggregate stability under fallow-maize rotation systems. The objectives of this study were to determine SMBC, aggregate protected carbon, and mineralizable carbon, SOC associated with sand, silt and clay and aggregate stability under conventional tillage (CT) and no-tillage (NT) in a fallow-maize rotation system. We hypothesized that improved fallow systems, which are fast growing leguminous trees grown during the fallow phase, had greater SMBC, aggregate protected carbon, SOC in the different soil fractions and aggregation when compared to natural fallow or continuous maize systems. Treatments studied were improved fallows of Acacia angustissima (A. Angustissima), Sesbania sesban (S. Sesbania), natural fallow and continuous maize. Samples were collected from 0–50, 50–100 and 100–200 mm depths 1 year after fallow termination in plots subjected to CT and NT. Total SOC was determined by wet combustion, SMBC by fumigation extraction, aggregate protected SOC by wet sieving and sedimentation into sand, silt and clay after 21-day incubations of intact and crushed macroaggregates of size

Effect of Tillage System on Distribution of Aggregates and Organic Carbon in a Hydragric Anthrosol

The effect of different tillage systems on the size distribution of aggregates and organic carbon distribution and storage in different size aggregates in a Hydragric Anthrosol were studied in a long-term experiment in Chongqing, China. The experiment included three tillage treatments: conventional tillage with rotation of rice and winter fallow (CT-r) system, no-till and ridge culture with rotation of rice and rape (RT-rr) system, and conventional tillage with rotation of rice and rape (CT-rr) system. The results showed that the aggregates 0.02–0.25 mm in diameter accounted for the largest portion in each soil layer under all treatments. Compared with the CT-r system, in the 0–10 cm layer, the amount of aggregates > 0.02 mm was larger under the RT-rr system, but smaller under the CT-rr system. In the 0–20 cm layer, the organic carbon content of all fractions of aggregates was the highest under the RT-rr system and lowest under the CT-rr system. The total organic carbon content showed a positive linear relationship with the amount of aggregates with diameter ranging from 0.25 to 2 mm. The storage of organic carbon in all fractions of aggregates under the RT-rr system was higher than that under the CT-r system in the 0–20 cm layer, but in the 0–60 cm soil layer, there was no distinct difference. Under the CT-rr system, the storage of organic carbon in all fractions of aggregates was lower than that under the CT-r system; most of the newly lost organic carbon was from the aggregates 0.002–0.02 and 0.02–0.25 mm in diameter.

Biomass production, nutrient uptake and partitioning in planted Senna spectabilis , Flemingia macrophylla and Dactyladenia barteri fallow systems over three fallow/cropping cycles on Ultisol

Biomass production and nutrient uptake of all components in planted Senna spectabilis, Dactyladenia barteri and Flemingia macrophylla hedgerow fallow were determined after three two‐year fallow phases and compared to natural fallow. Total above-ground biomass production after each of three fallow phases was significantly higher in the Senna system than any other system. Total above ground biomass production in the Flemingia and the Dactyladeniai system was not different from that in natural fallow. The volunteer biomass between hedgerows was only once significantly reduced by S. spectabilis during the first fallow phase. The amount of litter did not differ between fallow systems. The biomass of S. spectabilis, F. macrophylla and D. barteri comprised 96%, 95% and 65% wood, respectively. Relative to the total system biomass, wood constituted 67% in S. spectabilis and about 20% in F. macrophylla and D. barteri systems. Except for Mg, the S. spectabilis system accumulated more nutrients in above ground biomass than any other system. The highest nutrient uptake achieved in the S. spectabilis system was 335 kg ha−1 N, 331 kg ha−1 Ca, 230 kg ha−1 K, and 39 kg ha−1 P. Relative to the S. spectabilis system, nutrient accumulation, except for Mg, was the lowest in the natural fallow control followed by the F. macrophylla and the D. barteri system. In F. macrophylla and S. spectabilis, 95% and 85% of the nutrients were accumulated in the wood. In D. barteri the nutrient distribution between leaves and wood was approximately equal. Export of wood would remove 9–16% of the nutrients accumulated in the F. macrophylla and D. barteri systems but 27–53% in the S. spectabilis system. Potential consequences of different biomass management options for crop yields and fallow re-growth are discussed.

Quantifying soil water conservation in the semiarid region of Saskatchewan, Canada: Effect of fallow frequency and N fertilizer

Soil water is the most limiting factor influencing crop production in the semiarid prairies. The effects of fallow frequency and nitrogen (N) fertilization on soil water conservation were quantified for a 40-yr (1967–2006) field experiment conducted on a medium textured Orthic Brown Chernozem (aridic haploboroll) in semiarid southwestern Saskatchewan, in which soil water contents were measured each year in early spring (generally a week prior to seeding), shortly after harvest, and again just prior to freeze-up in the fall. The three treatments examined were continuous spring wheat (Triticum aestivum L.) (Cont W) and fallow-wheat (F-W), each receiving recommended rates of N and P fertilizer, and Cont W receiving only P. On average, 36% of the precipitation received during the fall and winter months for Cont W (N + P) was conserved in the soil. In the summer fallow system [F-W (N + P)] a greater proportion (42%) of the precipitation was conserved during the first fall and winter. Despite the fact that cumulative precipitation from spring to late fall during the fallow period averaged 243 mm, compared with 152 mm received during the previous fall and winter period, the amount (31 mm) and proportion (13%) of precipitation conserved was considerably less than that during the first overwinter period. These differences were attributed mainly to much higher summer evaporative losses. During the second overwinter period, only 6% of the precipitation received was conserved in the F-W (N + P) system compared with 44% in the first overwinter period. This poor conservation during the second winter was thought to be related to increased snow blowoff due to smaller amounts of standing crop residues, and to the freezing of a wet and bare soil surface, restricting water entry during snowmelt or spring thaw events. Physical based soil-crop models, which must be first improved for overwinter simulations, should be tested on the current data set to further interpret the observations and hypotheses. Compared with the 36% of fall and winter precipitation conserved in Cont W (N + P), inadequate N fertility [Cont W (+ P)] resulted in only 27% of the precipitation being conserved during this period. At harvest, F-W (N + P) and Cont W (N + P) had similar amounts of water in the soil, but Cont W (+ P) had significantly (P &lt; 0.05) more because of reduced water use. However, by the following spring soil water recharge being proportional to crop residues produced resulted in F-W (N + P), Cont W (N + P) and Cont W (+ P) having 252, 209 and 204 mm/1.2 m soil, respectively. Equations were developed that will allow estimation of water conserved as a function of precipitation received between harvest and seeding for F-W (N + P) and Cont W (N + P) (R2 = 0.52*** in each case). Trends in grain yield were fairly closely correlated with growing season precipitation and potential evapotranspiration [squared semipartial correlation coefficients for Cont W (N + P) 0.32 and 0.17, respectively, and for F-W (N + P) 0.35 and 0.12]; soil water conservation played a minor role in determining final grain yields (squared semipartial correlation coefficient &lt; 0.08). Key words: Soil water, spring wheat, precipitation deficit, yield trends, water conserved trends

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.

Organic material decomposition and nutrient dynamics in a mulch system enriched with leguminous trees in the Amazon

The new techniques proposed for agriculture in the Amazon region include rotational fallow systems enriched with leguminous trees and the replacement of biomass burning by mulching. Decomposition and nutrient release from mulch were studied using fine-mesh litterbags with five different leguminous species and the natural fallow vegetation as control. Samples from each treatment were analyzed for total C, N, P, K, Ca, Mg, lignin, cellulose content and soluble polyphenol at different sampling times over the course of one year. The decomposition rate constant varied with species and time. Weight loss from the decomposed litter bag material after 96 days was 30.1 % for Acacia angustissima, 32.7 % for Sclerolobium paniculatum, 33.9 % for Iinga edulis and the Fallow vegetation, 45.2 % for Acacia mangium and 63.6 % for Clitoria racemosa. Immobilization of N and P was observed in all studied treatments. Nitrogen mineralization was negatively correlated with phenol, C-to-N ratio, lignin + phenol/N ratio, and phenol/phosphorus ratios and with N content in the litterbag material. After 362 days of field incubation, an average (of all treatments), 3.3 % K, 32.2 % Ca and 22.4 % Mg remained in the mulch. Results confirm that low quality and high amount of organic C as mulch application are limiting for the quantity of energy available for microorganisms and increase the nutrient immobilization for biomass decomposition, which results in competition for nutrients with the crop plants.

Modelling of planted legume fallows in Western Kenya. (II) Productivity and sustainability of simulated management strategies

Improved fallow is a technology that can help to raise agricultural productivity in systems of poor soil fertility and low financial capital. Models, once calibrated, can be used to investigate a range of improved fallow systems relatively quickly and at relatively low cost, helping to direct experimental research towards promising areas of interest. Six fallow crop rotations were simulated using the WaNuLCAS model in a bimodal rainfall setting in Kenya over a 10 year period: (A) alternating fallow and crop seasons, (B) one season fallow followed by three seasons crop, (C) one season fallow followed by four seasons crop, (D–F) 1–3 seasons fallow periods followed by 3–5 seasons crop. The strategies were tested using a number of fallow growth rates, soil clay contents, and rainfall amounts to determine the interaction of fallow rotation and biophysical variables on maize (Zea mays (L.)) yield and sustainability (organic matter, N2 fixation, leaching). The best simulated fallow strategies doubled maize yield compared to continuous maize over a 10 year period. Across all biophysical treatments strategy A and B of no more than three consecutive cropping seasons and of one consecutive fallow season yielded the most maize. This was because fallow benefits were largely due to the immediate fallow soil fertility benefit (IFB) rather than the cumulative benefit (CFB). The difference in yield between the two strategies was through a balance between (1) their interaction with the biophysical variables affecting accumulation of organic matter, hence increasing soil fertility and (2) the extra intrinsic soil fertility used for maize productivity by the inclusion of more cropping seasons within the rotation. We propose the following conceptual framework to manage fallows for maximum maize yield: when environmental factors are strongly limiting to fallow and crop growth then fallow strategy A would be the best strategy to employ (less risk but more labour) and when factors are less limiting then strategy B would be the best to employ.

Planted legume fallows reduce weeds and increase soil N and P contents but not upland rice yields

Shortened fallows have resulted in declining upland rice yields in slash-and-burn upland rice systems in northern Laos. We studied the benefit of planted legume fallows for rice productivity, weeds, and soil nitrogen and phosphorus availability. Four systems were evaluated over a 5-year period: 1-year fallow with native species, 1-year Cajanus cajan fallow, 1-year Leucaena leucocephala fallow, and continuous annual rice cropping. Rice was grown either once each year as continuous annual cropping or in alternate years of 2001, 2003, and 2005. C. cajan and L. leucocephala were sown with rice during the 2001 growing season. In subsequent years, L. leucocephala regenerated from root stock and did not have to be resown, whereas C. cajan was resown in 2003. Establishment of either C. cajan or L. leucocephala had no significant effect on rice yield in 2001, and rice yields ranged from 2.0 to 2.3 t/ha. Rice yields declined rapidly in succeeding years, and rice yields in the four systems ranged from 0.7 to 1.1 t/ha in 2003 and from 0.3 to 0.5 t/ha in 2005. Although two planted fallow systems increased nitrogen input because of greater biomass accumulation in 2003 and 2005 and soil phosphorus availability was higher following L. leucocephala fallow in 2005, there were no significant differences in rice yields among the four systems in either year. Weed biomass during the rice growing season increased each year in all systems and increased more rapidly for continuous annual rice cropping, in which the dominant weed species was Ageratum conyzoides L. Among the other three systems, there were no significant differences in the weed biomass in 2003 and 2005. We conclude that C. cajan and L. leucocephala as 1-year fallows do not offset the negative effects of increased cropping intensity on rice yield in this region.

Soil property indices for assessing short-term changes in soil quality

AbstractSoil quality has been proposed as a prime indicator for characterizing and defining management factors contributing to soil degradation. In this study, biological (soil respiration, fluorescent Pseudomonas bacteria and entomopathogenic nematode populations), chemical (pH, inorganic N, and total C &amp; N), and physical (bulk density and infiltration) indicators were used to determine soil quality. The specific research objective was to determine the capacity of this specific set of indicators to assess soil quality and determine its ability to detect short-term changes in soil conditions and processes. The assessment was comparative because of the lack of specific criteria or guidelines available in the literature for interpretation of most soil property indices measured. The following treatments were chosen from an ongoing farming systems study to achieve a preplanned set of comparisons that would make this type of assessment possible: (1) best management practices/conventional tillage (BMP/CT), (2) BMP/no-tillage (BMP/NT), (3) an organic system, and (4) a successional fallow system. Assessments were made multiple times between 1999 and 2000. Statistical differences between systems were found for all soil quality indicators except for entomopathogenic nematodes. Differences between systems varied across dates, a result that supports other research stating the need to consider the temporal variability of these indices for an unbiased overall soil quality assessment. Differences in total carbon and total nitrogen between systems were most evident in the 2000 sampling dates with BMP/NT showing greater contents on the last sampling date. The soil pH and inorganic N results did not suggest a possible difference in soil function status between any of the three agricultural systems studied. All three agricultural systems, BMP/NT, BMP/CT and organic, had similar pH values and overall low soil inorganic N levels. The non-agricultural successional system had a slightly more acidic soil condition than the three agricultural systems. Soil bulk density increased with time in the untilled BMP/NT and successional systems but the resulting values were not considered detrimental to either productivity or environmental quality. Infiltration was lower in the BMP/NT and successional systems than in the BMP/CT and organic systems. In conclusion, all soil quality indicators except for entomopathogenic nematodes proved to be sensitive to the detection of rapid changes in soil conditions that occur by the influence of soil management. The importance of using soil bulk density to express soil results on a volume basis, as the soil exists in the field before sampling, prevented an average interpretation error of 7–14% as compared to treatment comparisons on a soil weight basis only. This also demonstrates the need to carefully consider field sampling locations (row, between row, or wheel traffic areas) which dramatically influence soil density, physical characteristics, organic matter concentrations, and biological activity. Failure to consider these factors can invalidate even the most careful approaches to establishing baseline soil quality levels in the field as affected by various tillage and residue management practices and associated comparisons in time.

Groundnut/Cassava/Maize Intercrop Yields over Three Cycles of Planted Tree Fallow/Crop Rotations on Ultisol in Southern Cameroon

ABSTRACT Lack of crop yield response to planted tree fallow led to introducing a two-year fallow phase to determine if planted tree fallow can improve soil fertility and yields over a no-tree control. Three cycles of two years fallow followed by slash-and-burn land preparation and one year of groundnut/cassava/maize intercropping were conducted with Senna spectabilis, Flemingia macrophylla and Dactyladenia barteri as planted hedgerow fallows and a no-tree control on an Ultisol in southern Cameroon. The land had been continuously cropped to maize/cassava intercrop for 5 years previous to the first two-year fallow phase. Groundnut grain yields were unaffected by fallow system in 1998 and 2001 and the sum of the three cropping years. Maize grain yield was unaffected by fallow system in 1998. In 2001 and 2004 maize grain yield was highest in the S. spectabilis system. Total maize grain yield across the three cropping years was higher in the F. macrophylla and S. spectabilis systems than in the D. barteri system. Cassava root yields were in all years and the sum of the 3 years unaffected by fallow system. Cassava root (1998, 2001) and groundnut grain (2001) yields had significant spatial responses to the distance from hedgerows, with yield increases with increasing distance from hedgerows. Annual biomass production of hedgerow prunings during cropping phases ranged from zero (D. barteri) to 3.4 Mg ha−1 (S. spectabilis). During the growth of groundnut and maize, hedgerows produced < 1 Mg ha−1 in 1998, < 0.6 Mg ha in 2001 and <0.8 Mg ha at any individual pruning. Combined relative crop yields over the three cycles were lower in planted fallow than in the no-tree control. The N export with groundnut and maize grain and cassava roots, as an indicator of crude protein production was lower in the planted hedgerow fallow systems than in no-tree control. The planted fallow hedgerow system appears unsuitable to improve crop yields because the nutrient supply from prunings is low due to their low biomass production. Yet on short distances, the spatial response of cassava and groundnut indicates competition between hedgerows and crops, which was most pronounced on cassava and groundnut in the S. spectabilis system. The crop combination appears incompatible as benefits realized by the maize were outweighed by losses in groundnut.

Soil Population of Helicotylenchus multicinctus under Pueraria phaseoloides, Flemingia macrophylla and Natural Bush Fallows and their Effect on Plantain Yield in the Humid Forest Zone of Southern Cameroon

ABSTRACT Soil population densities of Helicotylenchus multicinctus (Cobb) Golden were assessed under Flemingia macrophylla [(Willd.) Merrill] and Pueraria phaseoloides (Roxb.) Benth, and natural bush fallows over 24 months in three villages in southern Cameroon. Also assessed was the effect of H. multicinctus on yield of plantain. Trials were set up in four to five year-old Chromolaena odorata (L.) R. M. King & H. Rob. dominated natural bush fallow. Initial H. multicinctus population density at Ngoumou (512 100 cm−3 soil) was (p < 0.05) higher than at Mfou (241 100 cm−3 soil) and Nkometou (70 100 cm−3 soil). H. multicinctus population was correlated with (i) total annual rainfall (r = 0.79; p = 0.01) and (ii) clay content (r = 0.95; p < 0.001). At 12 months after planting (MAP), P. phaseoloides and F. macrophylla supported (p < 0.01) lower population densities of H. multicinctus (88 and 152 100 cm−3 soil, respectively) compared with the natural regrowth (287 100 cm−3 soil). Comparing the initial population with that at 12 MAP for each of the three fallow systems, both P. phaseoloides and F. macrophylla reduced density of H. multicinctus (p < 0.05). However, when the initial population density was compared with that at 24 MAP only under P. phaseoloides was the reduction maintained. Contrarily, population density under F. macrophylla was comparable between the initial and 24 MAP. This is the first report of the nematode reducing effect of P. phaseoloides. Plantain flowering and subsequent harvesting was earliest under P. phaseoloides, different from that under F. macrophylla (p < 0.05). Plant bunch weight was positively correlated with the length of the fruit filling period (r = 0.67) which in turn was positively correlated with the non damaged root index (NDRI) (r = 0.58). Plantain under P. phaseoloides had the highest NDRI and consequently recorded the greatest yield (p < 0.05). There was no difference in plantain yield between the natural regrowth and F. macrophylla. Pueraria phaseoloides could be used as one component for the biological control of H. multicinctus.

Effect of Mucuna Fallow on Succeeding Maize Nutrition and Yield in Farmer-managed On-farm Trials

ABSTRACT On 18 farmer-managed fields in the Forest-Savannah Transitional zone of Ghana, the response of unfertilized maize was investigated as influenced by either a previous short season natural fallow or Mucuna pruriens (L.) DC var. utilis (Wright) Burck. Comparative vector diagnosis of three weeks old seedlings revealed that N was limiting maize growth. The previous Mucuna fallow was at least partly able to compensate N while there was a dilution of all the other macronutrients due to an increased uptake in the Mucuna system. Maize yields were on average 51% higher after Mucuna fallow with 2850 kg ha−1 compared with 1880 kg ha−1 after natural fallow, but the variability between farmers' fields was high. Adaptability analysis revealed that the slash-and-mulch leguminous fallow system was most effective at locations of high productivity while the yield difference to the slash-and-burn natural fallow system was small in farming environments with low productivity. It was concluded that M. pruriens var. utilis fallow is not a suitable technology to reclaim degraded land or to restore soil fertility if growth-limiting factors are not eliminated. Low available P levels and later maize planting significantly reduced the yield difference and hence the risk for M. pruriens to be outperformed by the natural fallow system increased up to 41% with lower values of plant available P and later maize planting. Besides individual yield components such as cob number and cob seed weight, plant population at final harvest accounted for 58% of the variation in yield. Nutrient use efficiency improved significantly for all macronutrients except P but the harvest index was significantly lower at 0.40 after Mucuna fallow compared with 0.43 after natural fallow. N supplied by the Mucuna fallow supported a better growth of the succeeding maize crop but only in the beginning of the cropping season. When the yield level a farmer usually experiences with his common practice on a specific field was applied to the regression models of the adaptability analysis it was possible to make site- specific yield predictions for the M. pruriens system. It was concluded that the application of the adaptability analysis and individual risk assessments might serve as valuable tools to give more site-specific recommendations to farmers instead of blanket messages for entire regions.

Short- and Long-term Maize Yield Response to Mucuna pruriens and Pueraria phaseoloides Relay Fallow and Biomass Burning Versus Mulching in the Forest Zone of Southern Cameroon

ABSTRACT Maize was relay cropped with the cover crops Mucuna pruriens or Pueraria phaseoloides for between one and seven consecutive years in three sites: Ngoungoumou, located in an area of low land use intensity (LUI), about 100 km away from the next major market, and at Evendissi and Andok, in a medium LUI area, 15 to 20 km away from the next major market. Maize performance was compared with that in a natural fallow system, with fallow biomass burned versus retained as mulch in both fallow types. Fallow type had no effect on maize density. After biomass burning higher maize densities were attained at the low LUI site. Maize cob production was affected neither by fallow type nor by biomass management. Across seven years of cropping, marketable cob production in the low LUI site was 38.5% higher in mucuna fallow than in natural fallow (p ⁢ 0.06). Differences at p ⁢ 0.05 were found in one out of seven years. In the medium LUI sites, mucuna and pueraria fallow increased marketable cob production by 70–132% (p ⁢ 0.001). Maize grain yield was closely related to the marketable cob production in all sites. In the low LUI site the cumulative maize grain yield over seven years was 30% higher in mucuna fallow than in natural fallow (p < 0.07), the difference being significant at p ⁢ 0.05 in only one year. In the medium LUI sites maize grain yield was 65% higher in mucuna fallow (p < 0.001) and 69–94% higher in pueraria fallow (p ⁢ 0.001). Burning biomass had a cumulative grain yield advantage of 33% at the low LUI site (p ⁢ 0.053), with significant (p ⁢⁢ 0.02) differences in two out of seven years. Biomass burning had no effects on maize grain yield in the medium LUI sites. No interactions between fallow type and biomass management were found. Despite the lack of frequent significant advantages of the cover crop fallows in the low LUI site, the system can be recommended because it has the potential to increase food diversity and security; in the medium LUI sites farmers were immediately convinced of the benefits of the system and have since moved on to grow high value vegetable crops.

Tillage and the environment in sub-tropical Australia—Tradeoffs and challenges

Tillage is defined here in a broad sense, including disturbance of the soil and crop residues, wheel traffic and sowing opportunities. In sub-tropical, semi-arid cropping areas in Australia, tillage systems have evolved from intensively tilled bare fallow systems, with high soil losses, to reduced and no tillage systems. In recent years, the use of controlled traffic has also increased. These conservation tillage systems are successful in reducing water erosion of soil and sediment-bound chemicals. Control of runoff of dissolved nutrients and weakly sorbed chemicals is less certain. Adoption of new practices appears to have been related to practical and economic considerations, and proved to be more profitable after a considerable period of research and development. However there are still challenges. One challenge is to ensure that systems that reduce soil erosion, which may involve greater use of chemicals, do not degrade water quality in streams. Another challenge is to ensure that systems that improve water entry do not increase drainage below the crop root zone, which would increase the risk of salinity. Better understanding of how tillage practices influence soil hydrology, runoff and erosion processes should lead to better tillage systems and enable better management of risks to water quality and soil health. Finally, the need to determine the effectiveness of in-field management practices in achieving stream water quality targets in large, multi-land use catchments will challenge our current knowledge base and the tools available.

Farmyard Manuring, Tillage, and Irrigation Management Impacts on Forage Sorghum Grown in Rice Fallow Systems

ABSTRACT Field experiments were conducted from 2000 to 2003 in rainfed lowland rice systems at the Central Rice Research Institute, Cuttack, India to study the influence of applying farmyard manure (FYM) in preceding kharif rice on succeeding rabi forage sorghum with different tillage and irrigation management. After the harvest of rice treated either with FYM or no FYM, sorghum cultivar ‘SPV 1505’ was sown with zero tillage and conventional tillage under two levels of irrigation, recommended requirement and 50% of requirement. Results showed that crops produced consistently more green forage (60.6–66.7 t/ha), dry matter (11.0–12.1 t/ha) and crude protein yield (0.88–1.23 t/ha) over the years while raised under FYM-treated plot; however, no differences were found with different tillage methods and irrigation levels. Dry matter and crude protein yields also remained the same within these treatments. In all the years crude protein content was significantly higher in crops raised under FYM plots (8.0–10.2%) and recommended irrigation (0.90–1.09%), while crude protein (0.71–0.94%) in crops under zero tillage were comparable with those (0.74–0.89%) conventional tillage. From these results, the recommendations for farmers are to adopt zero tillage instead of conventional tillage, to reduce irrigation requirement by 50% of its requirement and to apply FYM in the preceding rice crops for sustainable forage sorghum cultivation in lowland rice fallow systems.