Grass Fallow Research Articles

Trees in agricultural landscapes maintain soil organic carbon following miombo woodland conversion to shifting cultivation

The main driver of deforestation and degradation of miombo woodland is shifting cultivation (SC). SC is the main farm system and livelihood of the people in miombo woodlands. Assessing the current body of knowledge, the following apparent gaps were identified: (i) researches analysing the response of soil-C to woodland (miombo) conversion to SC are contradictory; (ii) studies reporting all the stages of agriculture-induced ecological succession (woodland, shifting cultivation, fallow vegetation, and reconversion to shifting cultivation or to woodland) are lacking. The response of soil-C to (1) dry miombo woodland (DM) conversion to SC, (2) SC conversion to fallow vegetation, (3) reconversion of fallow vegetation to SC, (4) the length of the cropping and fallowing periods, and (5) land-use intensification, was studied. The study was carried out in a landscape mosaic consisting of treeless SC, tree-based SC, grass fallow (treeless SC-fallow), and woody fallow (tree-based SC-fallow). DM conversion to treeless SC resulted in depleted soil-C by up to 72%; the depletion increased with the increasing length of the cultivation period. The fallow period (grass fallow) recovered part of the soil-C lost during the cultivation period: grass fallow lands were approximately threefold larger in soil-C stocks than treeless SC lands. Soil-C increased with increasing grass fallow age. No significant changes in soil-C were observed when DM was converted to tree-based SC, or when tree-based SC lands were left under fallow (woody fallow); consequently, soil-C did not vary with the increasing lengths of cultivation and fallow periods. Woody fallows were up to 45% higher in soil-C stocks than grass fallows. Reconversion of grass fallow to treeless SC resulted in soil-C loss; no changes were observed when woody fallows were reconverted to tree-based SC. Land-use intensification by increasing the frequency of SC cycles did not alter soil-C in either treeless or tree-based SC. It was concluded that, under the study area conditions, the response of soil-C to agriculture-induced land-use changes depends mainly on the presence or absence of the arboreal component. Maintaining or introducing the arboreal component is recommended to avoid or mitigate soil-C loss following DM conversion to SC.

Impact of tree plantations on the dynamics of soil aggregates in urban degraded lands in the dry tropics

AbstractThe impact of land‐use change in an urban ecosystem on soil aggregate dynamics and soil carbon storage was studied through two annual cycles under five land uses. All the land uses namely: (1) natural mixed vegetation (NV) growing in a protected area; (2) grass fallow (GF); and tree plantations of (3) Cassia siamea (CP); (4) Jatropha curcas (JP); and (5) Tectona grandis (TP) were grown on urban degraded waste lands in a dry tropical region of India. Proportions of soil aggregate fractions, their stability, and soil carbon storage were analyzed at upper (00–15 cm), and lower (15–30 cm) soil depths during the rainy, winter, and summer seasons. Across all the land uses and at both the soil depths, the proportion of macroaggregate was highest during summer and lowest during the rainy season, whereas the reverse trend was observed for mesoaggregate. The annual mean proportion of macroaggregate decreased while meso‐ and microaggregates increased, with increasing soil depth across all the land uses. At the upper layer, the trends of macroaggregate, aggregate stability, and soil C storage were NV > CP > JP > TP > GF whereas at lower depth, the trend was NV > TP > CP > JP > GF. Across all the land uses in both the depths, soil C storage was strongly and positively correlated with annual mean fraction of soil macroaggregate. All tree plantations improved soil properties considerably as compared to GF at upper soil layer, yet TP reached near NV in these soil properties at lower depth. Soil aggregate fractions may be used as an index of carbon storage capacity in the urban ecosystems.

Effects of vegetative mulches on growth of indigenous crops in the Kingdom of Tonga

AbstractAs in many areas of the developing world, intensification of agriculture in Tonga, and other Pacific Islands, has put increased pressure on the soil resource. Two experiments were conducted to evaluate the effect of mulch on the growth and yield of two important food and fibre crops. The first was conducted on sloping land to evaluate the effect of guinea grass (Megathyrsus maximus) mulch and hedgerows on taro [Colocasia esculenta (L.) Schott] yield, and in controlling soil erosion. The second compared the response of paper mulberry [Broussonetia papyrifera (L) Ventenot] to different management regimes of a grass fallow. Thick vegetative mulch increased taro corm yield by 81% and reduced soil loss by 50% compared to local farmer practice, and the soil loss from taro with mulch was comparable to the perennial cash hedgerow treatment. Mulch increased paper mulberry bark yield by 30% compared to the non‐mulch control. Comparative economic analysis showed that increased net profit in the mulched treatments compared to the non‐mulched control was T$2660/ha for taro and T$12 108/ha for paper mulberry. Considering that mulch is readily available to many farmers throughout the Pacific Islands and elsewhere in the tropics, it is recommended as a sustainable practice for crop production.

Land Use Impact on Soil Physical Quality and Soil Structure in Three Highland Watersheds of Ethiopia

Continuous monitoring of soil physical quality is essential in determining sustainability of land use in natural and managed ecosystems particularly; mountainous systems such as the Ethiopian highlands, where soil deterioration and degradation can become major threats to ecosystem and productivity. This study focused on assessment of soil physical quality (SPQ) by comparing values of soil indicator properties to their corresponding ‘ideal ranges’ established in literature and determining soil structural stability and aggregate associated carbon as influenced by land use types. It further employs factor analysis to identify appropriate SPQ indicators and soil structural indices that are dominantly influenced by each land use type in the three watersheds (namely; Jeldu, Diga and Fogera) of the Ethiopian highlands. Surface soil samples were collected from four adjacent agricultural land use types (namely; Grass fallow, Tree fallow, Homestead and Continuous field cropping) within each of the three watersheds for study of Dexter “S” index, aggregate associated carbon and aggregate stability and other SPQ indicators. Results revealed that soils were only adequate in Relative Field Capacity and Plant Available Water Capacity in all watersheds. The soils of the watersheds still maintained good physical quality but there are strong indications of low soil structural resilience and tendency to degrade. Factor analysis grouped SPQ indicators and structural indices into three factors: (Factor 1) structural stability (with soil organic carbon, aggregate associated organic carbon and aggregate size being the most dominant soil quality indicators); no dominant attribute for Factor 2 and Factor 3 is mainly soil texture (with clay being the most dominant). Soil improvement programme in the watersheds lies in management practices that will promote proper management of organic carbon as it is the principal SPQ indicator that influenced variation in other soil attributes.

Carbon status and structural stability of soils from differing land use systems in the Kingdom of Tonga

AbstractMaintenance of soil carbon stocks is vital for the environment at large and for maintenance of soil chemical, physical and biological fertility. Tonga represents a country in agricultural transition from subsistence to commercial production and whilst this is good for the national economy the impact on soil resources is less clear. The major cropped soils, fallow vegetation types and forest systems of Tonga were identified in each island group and samples of representative soils (0.15 m depth) from each land use unit were taken. Total carbon (CT) and δ13C were measured and labile carbon (CL) determined by oxidation with 333 mm KMnO4. These data were used to determine the carbon management index (CMI) and the proportion of carbon from C4 species in the CT pool. Relative to primary forest, the soil CT and CL generally declined with changes in vegetation and more intense mechanical tillage. The contribution of C4 plants to soil C increased with intensity of mechanical tillage and the prevalence of C4 guinea grass (Panicum maximum Jacquin) fallow. The changes in soil C were reflected in the CMI, and CL was a more sensitive indicator of change than CT. These data indicates that all land use systems have experienced a large net loss of soil C relative to the forest systems. Soil mean weight diameter (MWD) decreased significantly with increased intensity of mechanical tillage and to a lesser extent with the intensity and length of cropping. The relationship between soil MWD and soil C was similar with soil CT and CL. Grass fallow was as effective as permanent vegetation systems in improving soil MWD and lowering the micro‐aggregate (<125 μm) fraction.

Structural Stability and Hydraulic Conductivity Of Nkpologu Sandy Loam Soil Under Different Land Covers in Southeastern Nigeria

Studies were conducted in the runoff plots at the University of Nigeria Nsukka Teaching and Resesarch Farm in 2010 and 2011 to monitor the changes in structural stability and saturated hydraulic conductivity (Ksat) of Nkpologu sandy loam soil under different cover management practices. The management practices were bare fallow (BF), grass fallow (GF), legume (CE), groundnut (GN), sorghum (SM), and cassava (CA) cultivation. Three samplings were carried out at the study site following the characterization of the soil at five- month interval marking the end of first cropping season, and the start and end of the second cropping season respectively. There was no change in soil texture due to treatments. The aggregate stability (AS), mean weight diameter (MWD), water dispersible silt (WDSi), aggregate size distributions (> 2 mm, 1-0.5 mm and < 0.25 mm) and Ksat showed significant (P =0.05) changes with time. Ksat varied (CV = 52%) significantly (P = 0.05) with a standard deviation (SD) value of 7.8. The highest values for Ksat, AS and MWD were obtained in the first sampling period whereas the lowest values were obtained in the last sampling period. There were significant effect (P =0.05) of cover management practices on AS, MWD and Ksat. The highest values for AS, MWD and aggregate size fraction > 2 mm were obtained under GF whereas the highest Ksat was obtained under GN. The lowest values for these parameters throughout the sampling periods were obtained under BF. The preponderance of aggregates < 0.5 mm under BF showed that raindrop impact and other agents broke down macroaggregates into microaggregates. The cover treatments generally increased organic matter (O.M.) content compared with the BF. The O.M. had significant correlation with two aggregate size ranges; 1-0.5mm (r = - 0.276* at P=0.05) and 0.5-0.25mm (r = - 0.245*at P =0.05). The study has shown that cover management practices affected the structural properties of the tropical sandy loam soil differently over time.Keywords: Aggregate stability, MWD, aggregate size distribution, macroaggregates, microaggregates, saturated hydraulic conductivity, Cover management.

Responses of Two Sweet Potato Varieties to Four Green Manure Sources and Inorganic Fertilizer in a Humid Tropical Ultisol

ABSTRACT Field experiments conducted from 1998 to 2000 are reported in which three legume green manure crops were studied for biomass production. The legume green manure crops and grass cover (control) were then turned into the soil as green manures and compared with inorganic fertilizer for production of two sweet potato varieties (Ipomea batatas (L.) Lam). Treatments comprised green manure legumes (Mucuna pruriens, Centrosema pubescens, Pueraria phaseoloides), natural grass cover (control) and inorganic fertilizer NPK. Sweet potato varieties were TIS 87/0087 and TIS 8164. All possible combinations of sweet potato varieties and the manure treatments were laid out in a randomized complete block design with three replications. Mucuna pruriens produced significantly higher biomass than Centrosema pubescens and Pueraria phaseoloides at 2 months after planting (MAP) in May-established green manure plots but not in August plantings. At 10 MAP in August establishment, however, Centrosema pubescens and Pueraria phaseoloides gave higher dry matter than Mucuna pruriens. Biomass production increased up to 3 MAP, with the dry matter obtained with Mucuna and natural grass cover being over 4 t ha−1 at 3 MAP in August establishment. Soil nitrogen was significantly higher with Mucuna than with other legumes and natural grass fallows. Sweet potato tuber yields obtained from Mucuna improved fallow were similar to those obtained with NPK fertilizer treatment, which was better than the yield with Pueraria and natural grass fallow. Sweet potato variety TIS 8164 produced significantly higher tuber yield than TIS 87/0087 in 1999 but not in 2000. Sweet potato tuber yields were higher with the two-year fallow than with one-year fallow by 176%.

Tree fallows: A comparison between five tropical tree species

The natural abundance of 15N and 13C, conventional soil analyses, and biomass production by maize were used to study the influence of five tropical tree species on soils and their fertility. The experiment was conducted in Morogoro, Tanzania, to compare Cassia (Senna) siamea, Eucalyptus camaldulensis, E. tereticornis (all non-N2-fixing), Leucaena leucocephala, Prosopis chilensis (both N2-fixing), and a grass fallow. Maize biomass production, which was correlated with N uptake (P=0.001), was higher on soils from plots with 5-year-old Leucaena and Prosopis spp. compared to the grass fallow, while other tree species had less favourable effects on maize growth. The per cent N was higher in soil and δ15N of soil total N was lower under Prosopis sp. compared to soil under other tree species, which suggests an input from N2 fixation by Prosopis sp. A transfer of fixed N to maize or to understorey grass species was, however, not indicated by the 15N natural abundance. Prosopis sp. contributed more C to the soil than the other four tree species; the difference in δ13C between soils from Prosopis sp. plots and from grass fallow plots showed that the tree contributed 11% to the total C of the soil over a period of 8 years. The leaves of the N2-fixing species had a low ratio of lignin+phenols to N, and maize growth was negatively correlated with this parameter. The Eucalyptus spp. had leaves with a high lignin+phenols to N ratio, contributed very little C to the soil, and lowered the soil pH.

Soil properties and crop performance on a kaolinitic Alfisol after 15 years of fallow and continuous cultivation

A long-term field experiment was established on a kaolinitic Alfisol in Ibadan, Nigeria, in 1972. The land was cleared manually from secondary forest and used for (i) continuous no-till cropping with maize (Zea mays L.) and maize/cassava (Manihot esculenta Crantz) intercropping, (ii) planted fallow of guinea grass (Panicum maximum Jacq.), leucaena (Leucaena leucocephala de Wit), and pigeon pea (Cajanus cajan Millsp.), and (iii) natural bush regrowth in a randomized complete block design with three replications. At the end of 15 years, the fallow plots were cleared manually and cropped with maize for three years. The chemical and physical soil properties and crop performance of the newly-cleared plots were compared with those under 15 years of continuous cultivation. A total of 26 woody species were identified on the bush regrowth plots. Above-ground biomass accumulation of the bush plots was 157 Mg ha-1 containing 1316 kg N ha-1. Guinea grass, leucaena and natural bush regrowth plots had comparable organic C concentrations (approximately 20 g kg-1) in the surface soil (0 to 10 cm) after 15 years. The organic C concentration in the surface soil under pigeon pea was the lowest (9.5 g kg-1) among the four fallow treatments. Soil under 15 years of continuous no-till maize with and without residue mulch, respectively, contained approximately half (10 g kg-1) and a quarter (5.7 g kg-1) of the organic C under natural bush or guinea grass fallow. The levels of exchangeable Ca, K, Mg and effective cation exchange capacity (ECEC) were lower in the soils under continuous cultivation than in those under natural bush and planted fallow. Soil acidification occurred in soils under continuous cropping as depicted by the lower pH values and greater exchangeable Al and Mn concentrations compared to the fallow plots. Grain yield of maize (3 to 5 Mg ha-1) without fertilizer application in the plots newly cleared from natural bush, guinea grass and leucaena fallow was comparable with that of continuous no-till maize with residue mulch and chemical fertilizer (N, P, K, Mg, Zn) applications. Among the four fallow treatments, maize grain and stover yields were the lowest in plots cleared from pigeon pea fallow.

Population and agricultural intensity in the humid tropics.

A review of models of agricultural intensification and their application to the agricultural systems of the humid tropics is presented. Taken into account are the distributions of these systems at various population densities, available data on labor efficiencies, the costs of establishing continuous cropping, and data regarding soils under cultivation and various types of fallow. The findings that fallows much longer than 10 or 15 years serve no known agronomic function, that given preindustrial technology, grass fallows are disadvantageous, even environmentally destructive, and that continuous cropping usually entails a considerable amount of environmental modification support the interpretations that agricultural intensification in the humid tropics is best understood in terms of ecologically optimal strategies at different population densities. Points needing further investigation are highlighted: the reasons for very long fallows, and the comparative labor efficiencies of fallow and continuous cropping systems where crops and environments are similar.