Mineralizable Nitrogen Pools and Associated Enzymatic Activities in Soils Amended with Biochar Manure Co-Compost

Abstract

Biochar-manure co-compost (BMC) produced from the addition of biochar to nitrogen (N) rich materials has increasingly been promoted as an organic amendment to improve fertility constraints in soils. However, studies on the composition of organic N (ON) pools, N releasing capacity and associated microbial and enzyme activities of BMC-amended soils are lacking. In this study, the effects of co-composted market wastes (MW), cattle (CM) and poultry (PM) manures, alone (manure compost-MC), or in combinations with rice husk biochar (RB), sawdust biochar (SB) and coconut husk biochar (CB) on the dynamics of N mineralization, soil microbial biomass (SMB) carbon (C) (Cmic) and N (Nmic), amidohydrolase activities (AA) (L-Asparaginase, L-Glutaminase and Amidase) distribution and changes of ON fractions and their relationships were investigated in three soils, namely Adentan, Denteso and Keta series. The soils were amended with 200 mg N kg-1 of the BMCs and MCs, incubated at 30 oC and periodically leached for 26 weeks. Using the first-order kinetic model, the potentially mineralizable N (No) and the mineralization rate constant (k) were obtained. The AA, Cmic, Nmic and selected chemical properties such as pH, total carbon (TC), organic N (ON), permanganate oxidizable carbon (POXC) of the BMC and MC-amended soils were also determined. Similarly, the distribution and changes in ON fractions namely: Total Hydrolysable N (THN), Hydrolysable Ammonia N (HAN), Hydrolysable Amino Sugar N (HASN), Hydrolysable Amino Acid N (HAAN), Hydrolysable Unknown N (HUN) and Non-Hydrolysable N (NHN) in the amended soils were also determined. The results showed that No values ranged from 120 to 205, 53 to 170 and 71 to 174 mg kg-1 in Adentan, Denteso and Keta series, respectively, with SB and CB-based compost recording the lowest No values, probably due to immobilization and sorption of liable N. The k values were lowest in BMCs compared to MC due to C and N stabilization. Except for TN and ON, all the chemical and biochemical properties (lignin, cellulose, C/N ratio, NH4+-N) correlated significantly with No. Except for ON, BMC had a higher influence on the chemical and microbial properties of the soils compared to MCs due to the initial ON of the MCs. The RB, CB and CM-based BMCs had the highest influence on SMB and AA, probably due to the increased labile C and N in the amended soils. Also, the AA and SMB were positively and significantly correlated with each other and with No (r ≥ 0.229; p ≤ 0.05) as well as with chemical properties (TC, ON, pH, POXC) of amended soils. Soils amended with BMC and MC had higher ON fractions compared to soils alone which was attributed to an increase in the extractable organic N by the amendments. A negative relation between the THN, HAN, HAAN and No indicates that these fractions contributed to No of the amended soils. A stepwise regression analysis showed that the reduction in HAN, HAAN and THN explained about 60% of the variations in No. A positive correlation between HAAN, HAN, THN and AA in BMC-amended soils suggests that the enzymes were hydrolyzing these organic N fractions to release inorganic N. It was concluded that BMC moderated the N releasing capacity of the soils in addition to enhancing the microbial properties and organic composition of the amended soils.