N₂O, CH₄ And CO₂ Emissions From Acidic Soils Affected By Dolomite Application

Soil acidification is one of the main problems of agricultural soils since it constraints agricultural productivity. Approximately 30% soils of the world are under the influence of acidity. In China, about 2 million km2 tropical and sub-tropical region is occupying acidic soils. To counteract soil acidification, practices of lime and dolomite application are usually adopted in agricultural systems. Application of lime or dolomite to acidic soils can trigger several processes associated with carbon (C) and nitrogen (N) in soils, which in turn affect the greenhouse gas (GHG) emissions in the atmosphere. Thus, the mechanisms of GHG emissions from acidic soils are extremely important and urgent. To understand the effects of dolomite [(CaMg(CO3)2] application on GHGs emissions from acidic soils, we conducted several laboratory studies. It is hoped that the results could provide scientific basis for exploring the important fundamental scientific and technological issues relevant to GHG emissions from acidic soils following dolomite application.Acidic soils, rapeseed-rice soil (PR soil, pH 5.25) and rice-fallow/flooded soil (PF soil, pH 5.52) were collected from Xianning city of Hubei province, China. Air-dried soils were treated with different doses of dolomite (0,1 and 2 g kg-1 soil, CK, L and H, respectively) and incubated at constant temperature of 25C° in the chamber. Different soil moisture regimes (55% and 90% water filled pore space (WFPS)), N fertilizer applications (0 and 200 mg N kg-1 as (NH4)2SO4)), dicyandiamide (0,1 and 2 mg kg-1 soil, DCD0, DCD10 and DCD20, respectively) and crop straw (rice and green bean straw) in combination of dolomite application were also investigated. Soil sub-samples were taken during all the experiment at regular intervals to analyze soil properties and gas concentrations. Soil pH was determined using pH-meter, ammonium (NH4+-N) and nitrate (NO3--N) concentrations were determined colorimetrically using auto analyzer (Seal, Germany), microbial biomass carbon (MBC) and dissolved organic carbon (DOC) were analyzed using TOC analyzer, concentrations of gases (nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2)) were analyzed using gas chromatography. The main results and conclusions are as following.Application of dolomite significantly increased soil pH of acidic soils with the different initial pH. Increment in soil pH was rapid at the early stage (within 1 to 17 days) of the incubation period and afterwards slow and steady increase was observed throughout period. The effects of higher dolomite application doses on soil pH change were stronger in soils. The soil pH in dolomite treated alone ranged from 5.30 to 7.70 in L and H treatments, respectively. Increasing soil moisture increased soil pH and higher rise in soil pH was observed in 90% WFPS as compared to 55% WFPS. Incorporation of crop residues also raised soil pH and higher pH was observed in green bean treated soil as compared to rice straw. Green bean straw had C/N ratio of 17.59 while rice straw had C/N ratio of 36.60. Application of dicyandiamide did not significantly influence soil pH. Highest soil pH level was under 90% WFPS in H dolomite treatment without N fertilizer application. However, N fertilizer application declined soil pH to some extent.PF soil showed higher concentrations of ammonium (NH4+-N) as compared to PR soils because of high native N contents. Application of dolomite rapidly increased NH4+ N concentrations and H dolomite generated higher concentrations as compared to L treatment. Ammonium concentrations were usually highest between 11 to 31 days of the incubation and after that decreased steadily until the end of the study. Application of N fertilizer and increasing soil moisture increased NH4+-N concentrations. Incorporation of crop straw also increased NH4+-N concentrations and green bean straw generated higher NH4+-N concentrations as compared to rice straw. Application of DCD inhibited nitrification and thus higher NH4+-N concentrations were observed in DCD treated soil. DCD20 retained higher NH4+-N concentrations in the soil as compared to DCD 10 indicating high efficacy of inhibiting nitrification in DCD20 treatment.Soil nitrate continuously kept increasing with the time indicating conversion of NH4+-N to NO3--N and nitrification. Generally, highest NO3--N concentrations were found at the end of the study period. Application of dolomite increased soil NO3--N concentrations and H dolomite was more effective to generate NO3--N concentrations as compared to L treatment. Application of N fertilizer and increasing soil moisture increased NO3--N concentrations. Incorporation of crop straw also increased NO3--N concentrations and green bean straw generated higher NO3--N concentrations as compared to rice straw. Application of DCD showed low NO3--N concentrations because of inhibiting nitrification rates.Soil microbial biomass carbon (MBC) and dissolved organic carbon (DOC) increased by dolomite application and higher contents of MBC and DOC were found in H treatment. Increasing soil moisture increased MBC and DOC contents and therefore higher contents were under 90% WFPS as compared to 55% WFPS. Application of N fertilizer and DCD slightly increased MBC and DOC contents but difference were not significant between+/-N and+/-DCD application treatment. Soil MBC and DOC were generally higher at the onset of the incubation and then declined with the time. Incorporation of crop straw increased MBC and DOC, and green bean straw addition showed higher DOC and MBC in the soil.Soil N2O emissions were usually highest at the commencement of experiment and then gradually decreased with the time of incubation. Increase in soil moisture contents created anoxic soil conditions which favored N2O production and thereby higher magnitudes of N2O emissions occurred under 90% WFPS as compared to 55% WFPS. Application of N fertilizer stimulated N2O emissions and generally peaked at 3rd day of the study. Highest N2O emission was 0.59 μg N20-N kg-1 h-1 under 90% WFPS in fertilizer applied soil. Incorporation of crop straw enhanced N2O emissions and higher emissions released from green bean straw added soil when compared to rice straw incorporated soil. N2O emissions in straw incorporated soil ranged from 0.31 to 4.32 μg N2O-N kg-1 h-1. N2O emissions decreased with dolomite application and magnitudes were lower in the H dolomite treatments as compared to the L treatments. Soil pH showed negative relationship with N2O emissions throughout the study period. Application of DCD also substantially reduced N2O emissions and rate of reduction increased with application dose of DCD. Application of DCD inhibited nitrification rates which lowered N2O emissions. Higher reduction was observed in DCD20 as compared to DCD 10 treatment. Lowest N2O emissions were observed in the combined application of DCD and dolomite.Application of dolomite increased soil CH4 uptake and decreased emissions. H treatment was stronger to increase rates of CH4 uptake. The increase in CH4 uptake was higher at the later stage of the incubation. High moisture contents of 90% WFPS showed emissions of CH4 instead of uptake by soil as compared to 55% WFPS. The increase in soil moisture causes obstruction for oxygen (O2) exchange and anaerobic soil conditions and resultantly production of CH4 rather than uptake. Application of N fertilizer enhanced CH4 emissions. However, dolomite and DCD application decreased CH4 emissions. Soil pH showed negative relationship with CH4 emissions throughout the study period.Application of dolomite substantially increased soil respiration by increasing soil pH. Soil CO2 emissions were usually highest at the beginning of the incubation and decreased with time with the decreasing contents of DOC and MBC in soil. The increment in soil pH increased solubilization and mineralization of organic matter locally, which acted as substrates for activating microbes. Dolomite as such contains inorganic carbon, which upon dissolution provided C in the soil and thereafter released into the atmosphere. An increase in soil moisture increased DOC and MBC and resultantly enlarged CO2 emissions. Application of N fertilizer also increased CO2 emissions but differences were not significant as compared with un-fertilized soil. DCD application did not show any steady effects on soil CO2 emissions. Crop straw incorporation significantly enhanced CO2 emissions and higher amount of CO2 released from green bean added soil because of easily available organic C in the soil.Overall, results showed that application of dolomite increased soil pH and therefore rehabilitation of soil acidification. The increase in soil pH triggered soil organic C and mineral N which subsequently influenced N2O, CH4 and CO2 emissions from acidic soils. Application of dolomite mitigated N2O and CH4 emissions, while increased CO2 emissions in acidic soils. Therefore, results suggest that application of dolomite is a good practice to counteract soil acidification as well as to reduce N2O and CH4 emissions, while it enhanced CO2 emissions.