Evaluating Effects of Pyrogenic Organic Matter and Biochar Amendments on Soil Structure, Soil Chemical Properties and Soil Fauna Dynamics in Tropical Agroecosystems

Abstract

During vegetation clearance to cultivated lands, charcoal making eliminates canopy effects on soil associated with trees while at the same time creating new conditions in and around such charcoal-making spots due to increased concentration of pyrogenic organic matter (PyOM). It is unclear how and/or whether such (un)intentional management practices affect the abundance, diversity and distribution patterns of soil macrofauna. However, biochar has been proposed as an amendment for improving soil fertility as well as a means of sequestering C into the soil. Nonetheless, biochar effects on soil macrofauna has received little attention despite the profound role they play in soil ecosystems. Therefore, the objectives of this work were to: i) assess effects of Croton megalocarpus and Zanthoxylum gilletii trees on the abundance, spatial distribution of soil macrofauna, and soil aggregation, ii) assess influence of converting these trees to PyOM on spatial distribution of soil macrofauna, iii) evaluate influence of PyOM from these trees on earthworm cast production in a mesocosm study, and iv) evaluate potential of biochar and fertiliser + biochar blends in restoring fertility of a nutrient deficient soil and the effects on soil fauna abundance and diversity. Biochar used in this study was prepared from Prosopis juliflora, a common invasive shrub found in the semi-arid parts of Kenya. In the field study, soil macrofauna samples were collected at increasing distances from the tree stems and the centre of charcoal-making spots. In the greenhouse study, earthworm casts were collected after every two days and at the end of experiment (at 30 days) as a measure of earthworm activity. In the on-station trial, soil and soil fauna samples were collected six and eight weeks after crop emergence. Highest soil macrofauna abundance was found below the tree canopies than away from the tree, though the trends differed with specific macrofauna group. Earthworms were prominent under the canopy of Z. gilletii, whereas beetles occurred in higher numbers under C. megalocarpus. Soil aggregate analysis showed higher small macro-aggregates and micro-aggregate fractions under the canopy of Z. gilletii. However, C content in these two aggregate fractions decreased by more than 50% in soils with longer duration of cultivation, with the greatest magnitude of differences under xiii the canopy of Z. gilletii. In contrast to trees, soil macrofauna (with the exception of centipedes) declined with distance from charcoal-making spots, with most notable trends in spots where Z. gilletii was used in charcoal making. The number of centipedes decreased with increasing distance from the centre of spots rich in Z. gilletii PyOM. Beetles, termites and crickets were significantly higher in spots rich in C. megalocarpus PyOM than Z. gilletii PyOM, though sampling distance had no significant influence. The mesocosm study showed that the weight of earthworm casts declined by as much as 30% on mesocosms with PyOM compared to the control. In the on-station field study, application of biochar and fertiliser + biochar blends led to more than 70% increase in C, N, available P and exchangeable K compared to the control, though these effects lasted only for a season. There were no significant difference between soil treated with biochar or fertiliser + biochar. The population of earthworms observed in plots treated with P. juliflora biochar increased with increasing amounts of biochar, whereas a decline in nematodes population, particularly bacterivores occurred in plots which received biochar, regardless of the amounts. The study shows detrimental effects of converting trees to PyOM, especially the endogeic earthworms which are known to rely heavily on soil organic matter for nourishment. This could perhaps be caused by high recalcitrance of PyOM compared to the litter from trees and root biomass, which are more palatable. Other soil macrofauna may not directly be influenced by PyOM since PyOM was not the main subtrate food substrate. Application of biochar resulted in an increased content of major soil nutrients, which shows that P. juliflora biochar can potentially be a valuable soil amendment for improving nutrients in soil that are severely deficient in N and P and low in organic matter. Blending fertilisers with biochar however, seems not to have had much effects in terms of soil nutrient retention as it was hypothesised. Further research with long-term application of biochar could be of great benefit in expounding their impacts on soil fertility and soil fauna, since seasonal variations could have affected the observed results at short-term scales.