Soil organic matter distribution as influenced by enchytraeid and earthworm activity

Loam and sandy soils, and the earthworm casts produced with 14C-labelled plant material in both soils, were incubated in airtight glass vessels with and without enchytraeids to evaluate the effects of soil fauna on the distribution and fragmentation of organic matter. After 1, 3, and 6 weeks, the amount of C mineralised was determined in soils and earthworm casts, and the soil was fractionated into particulate organic matter (POM), the most active pool of soil organic matter, after complete physical dispersion in water. The percentage weight of fine fractions (0-50 ?m) was 67.4␒n the loam soil. Sand (coarse, i.e. 150-2,000 ?m and fine 50-150 ?m) represented 87.2␘f total weight in sandy soil, while the percentages of C (PC) were 23.2␒n coarse POM (2,000-150 ?m) and 11.9␒n fine POM (150-50 ?m). These percentages were higher than those in loam soil, i.e. 3.4øcoarse POM) and 5.4øfine POM). The PC in coarse POM (9.50€and fine POM (16.4€remained higher in casts from sandy soil than in casts from loam soil (4.7␒n coarse and 14.3␒n fine POM). The highest percentages of 14C-labelled leaves were found in fine fractions, 55.9␒n casts from loam soil and 48.8␒n casts from sandy soil. The C mineralisation of the added plant material was higher in casts from the sandy soil (20.3€than from the loam soil (13.5Ž Enchytraeids enhanced C mineralisation in the bulk sandy soil, but did not affect the mineralisation of added plant material in either soil. The main enchytraeid effect was enhancement of the humification process in the bulk sandy soil, the casts from this soil, and the bulk loam soil

Chemical properties and soil organic matter assessment under fallow systems in the forest margins benchmark

Fallow management plays a key role in supplying nutrients to the growing crop, suppressing weeds, reducing pests and diseases, and improving the sustainability of cropping systems in the humid forest zone of West and Central Africa. We evaluated soil nutrient concentration, soil acidity and soil organic matter status under natural Chromolaena fallow, fallow without Chromolaena (where Chromolaena was removed by hand), and planted Pueraria fallow in three different sites with different soil physico-chemical properties in southern Cameroon. Human population pressure is high and fallow length is short in the northern site, while an intermediate position and the contrary were observed in the central and southern sites, respectively. The highest nutrient concentrations (Ca2+ and Mg2+), and effective cation exchange capacity (ECEC) were found in the sandy clayey soils in the northern site. The lowest Al saturation and soil acidity were noticed in the sandy soils in the central site, while in the clayey soils in the southern site the lowest nutrient concentration, ECEC, and the highest soil acidity and Al saturation were found. Pueraria decreased soil acidity and increased nutrient concentration relative to Chromolaena in the northern site. The beneficial effects of Pueraria on soil acidity and nutrient concentration became less pronounced in the central site and were absent in the southern site. In the northern and central sites, the highest maize yield was observed under Pueraria, showing the responsiveness of maize to N. In the soils of the southern site with the smaller weight (<1.50 mg per g of soil) of particulate organic matter (POM) (53–4000 μm) and greater chemical constraints (high soil acidity and Al saturation), higher in N content in medium (250–2000 μm) and fine (53–250 μm) POM was found under Pueraria compared to natural fallow. N content in medium POM under Pueraria was greater by 47% compared to the natural fallow. The N content in fine POM under Pueraria was greater by 59% relative to the natural fallow. In the southern site, chemical constraints induced very low maize yields under all fallow types. It appears that POM quality, i.e. N content, was more important for maize than POM quantity, i.e. weight, while groundnut and cassava were more sensitive to POM quantity. Pueraria have a beneficial ecological and environmental sustainability in the soil with low acidity, while Chromolaena seems to be more sustainable than Pueraria in the acidic soils. However, crop yields found in this study were low. Maize yield was lower than 2000 kg ha−1, groundnut grain dry matter yield was lower than 600 kg ha−1 and cassava was lower than 400 tuber dry matter kg ha−1. Therefore, to attain, highest productivity levels it is necessary to combine fallow management (primarily planted such as Pueraria which can increase N content even where there are chemical constraints or soil degradation) with the use of fertilisers (organic and mineral)

Comparative study of soil properties under Chromolaena odorata, Pueraria phaseoloides and Calliandra calothyrsus

Fallows improve soil fertility and allow sustainable agriculture. Soil fertility was assessed under different types of fallow through pH, nutrient concentrations and particulate organic matter (POM) quantity and quality. The two year-fallows were under Chromolaena odorata, Calliandra calothyrsus and Pueraria phaseoloides on a Typic Kandiudult. Soils were sampled from 0–10 cm and 10–20 cm depth. The weight of POM was 2 mg g−1 of soil under Calliandra, 3.9 mg g−1 under Chromolaena and 3.7 mg g−1 under Pueraria in the 0–10 cm layer. The tPOM-C (proportion of C in the total POM) and tPOM-N (proportion of N in total POM) were 26.1% and 14.5% under Calliandra, 39.6% and 18.8% under Chromolaena and 37.0% and 16.7% under Pueraria. However, despite the improvement of soil fertility under Pueraria as compared to planted Calliandra, the effect of Pueraria on nutrient concentration and POM status remained similar to that of Chromolaena. Calliandra increased soil acidity and allowed a deterioration of nutrient concentration (Ca, K), ECEC and an impoverishment of POM status

Carbon decomposition of the topsoils and soil fractions under forest and pasture in the western Brazilian Amazon basin, Rondonia

International audienceThe topsoils of two sites, comprising natural forest and 4- and 20-year-old pastures, respectively, were selected in Rondônia to evaluate the changes of soil organic matter due to pasture establishment. These changes were evaluated by measuring the proportions of the C and N associated with clay and silt fractions, and by the C decomposition (CD) rate of the whole topsoils and their size fractions. The topsoils studied had large proportions of C and N associated with fine fractions, especially with clay fractions. The CD rate of the silt fractions was higher than that of the clay fractions under the two forest topsoils and under the 20-year-old pasture. The CD rate of the silt fractions under forest vegetation at each site was significantly higher than that of the silt fractions under pasture vegetation at the same site. The CD of clay fractions followed the same trend as the silt fractions, showing an improvement in the stability of C associated with clay and silt fractions under pasture vegetation

Factors influencing organic carbon decomposition of topsoils from the Brazilian Amazon Basin

The influence of texture and local climate, especially precipitation, on C decomposition was evaluated in topsoils under forest and pastures at different sites located in the western, central, and eastern Brazilian Amazon Basin. Independent of the geographical location and vegetation cover, C decomposition was lower in topsoils of fine texture with a sand/clay ratio of less than 1, compared to topsoils of coarse a texture with a sand/clay ratio ranging from 2 to 8. In the former, the low sand/clay ratio appears to reduce C decomposition. In the latter, besides the effect of the high sand/clay ratio, the local climate, especially the annual precipitation, seems to play an important role in controlling C decomposition

Factors influencing carbon decomposition of topsoils from the Brazilian Amazon Basin

International audienceThe influence of texture and local climate, especially precipitation, on C decomposition was evaluated in topsoils under forest and pastures at different sites located in the western, central, and eastern Brazilian Amazon Basin. Independent of the geographical location and vegetation cover, C decomposition was lower in topsoils of fine texture with a sand/clay ratio of less than 1, compared to topsoils of coarse a texture with a sand/clay ratio ranging from 2 to 8. In the former, the low sand/clay ratio appears to reduce C decomposition. In the latter, besides the effect of the high sand/clay ratio, the local climate, especially the annual precipitation, seems to play an important role in controlling C decomposition

Characterization of organic matter in topsoils under rain forest and pasture in the eastern Brazilian Amazon basin

International audienceIn topsoils under forest and 7-, 12- and 17-year-old pastures, organic matter was characterized by analysing C and N distribution in particle-size fractions, the C decomposition rates of soil and particle-size fractions and by employing density-fractionation of macro-organic matter (>150 μm). The C and N associated with clay fractions increased with increasing age of pasture. The weight (%) of macro-organic matter and its heavy fractions (>1.37 g cm–3) also increased with increasing age of pasture. However, in a long-term incubation (100 days), these changes seemed to involve an increase in the C decomposition rate in the topsoil of the oldest pasture. Using the C decomposition rates of particle-size fractions, it appeared that silt and clay contributed differently to C decomposition in the whole soil. C associated with silt contributed to the C decomposition rate during the first 40 days of incubation, while C associated with clay contributed to C decomposition in the long-term incubation (after 40 days), especially when the clay fraction appeared to reach saturation point with respect to its ability to bind organic compounds and thus protect the soil from C loss