Effect of Biochar Amendment On Ph, Selected Nutrients Availability and Greenhouse Gas Emissions in A Paddy Soil from Yala Basin Under Anaerobic Conditions

Abstract

Although tropical paddy soils have high levels of phosphorous (P), most of it is in forms that are not available for crops. Water logging of paddy soils during rice cultivation leads to anaerobic conditions which could limit the availability of nutrients, alter soil pH and cause greenhouse gas (GHG) emissions. Biochar use for soil amendment is increasingly gaining fame due to its potential benefits of increased nutrient availability and reduced GHG emissions. However, information on the effect of biochar amendment on nutrient availability, soil pH and GHG emissions in tropical paddy soils are scanty. The aim of this study was therefore to establish the effect of biochar amendment on soil pH, phosphorus fractions, nitrogen, organic carbon, and greenhouse gas emissions under anaerobic conditions. A paddy soil collected from the Yala basin of Lake Victoria was used as a representative tropical paddy soil amenable to water logging. Four different treatments including S; unamended soil, SB; soil amended with biochar in the ratio of 99:1, SP; soil amended with KH2PO4 in the ratio of 99:1, and SPB; soil amended with biochar and KH2PO4in the ratio of 98:1:1 (w/w) respectively were prepared, water logged and incubated at 25oC in airtight glass containers. Air samples and soil aliquots were sampled periodically. Dissolved organic carbon, P and N were extracted and determined using UV/Vis spectrophotometry, total organic carbon by loss on ignition, pH using a pH meter and GHGs (CO2, CH4 and N2O) using gas chromatography. The data obtained was subjected to ANOVA, regression analysis and correlation analysis using Microsoft excel 2007. Data obtained indicated that biochar amendment increased different soil parameters in the ranges total P (500.11±34.38 - 1709.51±101.40 µg/g of soil), iron bound P (111.402±11.80 - 174.75±9.78 µg/g of soil), initial soil pH (3.96±0.15 - 5.00±0.12), and CH4 emission (7.52 x 10-6 - 2.33 x 10-5 µg/g of soil), reduced CO2 emissions(9.54±3.54 - 5.96±2.17 µg/g of soil), and reduced loosely sorbed P (66.11±4.93 – 49.10±2.30 µg/g of soil) and had no effect on N2O emissions, TOC, DOC, TON, ammoniacal N and organic N. P amendment increased total P(500.11±34.38 - 1001.98±30.34 µg/g of soil), loosely sorbed P (66.11±4.93 - 106.65±5.38 µg/g of soil), labile organic P (89.42±13.15 - 179.34±10.67 µg/g of soil), soil pH (5.22±0.1 - 5.94±0.07) and cumulative N2O emission. It however, reduced total N (1987.83±345.30 - 502.00±2.36 µg/g of soil), Organic-N (1695.91±118.56 - 340.39±4.30 µg/g of soil), CO2 (9.54±3.54 - 6.97±2.21 µg/g of soil), and CH4 emissions (2.33 x 10-5 - 1.14 x 10-5 µg/g of soil) with respect to SB but had no significant difference (p≤0.05) on TOC, DOC, TON and ammoniacal N. The high CO2/CH4 ratio (>2) showed dominance of fermentation over methanogenesis. Biochar amendment, in this study helped stabilize soil nutrients and reduced GHG emissions with the exception on methane although its levels were far much lower than for the carbon dioxide and nitrous oxide. The use of biochar for soil amendment (1% w/w) is thus commendable and would benefit the residents of Siaya County, policy makers and the physical environment