Effects Of Manganese Oxide On Acidic Soil Nitrification

Nitrification varied largely in acid soils,which is not fully explained by reported abiotic or biotic factors such as soil pH,nitrogen?N?availability or microbial community structures.Minimal information is available on the interactions betweenmanganese oxide?MnO₂?and N transformation,especially in the subtropical regions of Chinawhereheavy N fertilizers were applied,with high N cycling and rich in Fe/Mn oxides in soils.Manganese oxide,an important absorbent,catalyst,and redox agent in soils,may play a crucial role in soil nitrification.The reactions of Mn oxides in soils are similar to those of Fe.It is hypothesized that Mn oxides play important roles in regulating N cycling.To understand the mechanisms of nitrification variation,we studied the effects of MnO₂ on nitrification in acidic soils with different pH,soil moisture contents,and organic matter?SOM?amendment.The nitrification process was simulated by kineticmodel.Dynamic changes of amoA gene abundance for ammonia-oxidizing bacteria?AOB?and ammonia-oxidizing archaea?AOA?were determined using qPCR.The major results are summarized as follows:1.Typical acidic agricultural soils with different soil pH were selected and amended with 0%or 3%MnO₂.Results showed that nitrification in weakly acidic soil was retarded by the additional of MnO₂,as evidenced by significantly decreased maximum nitrification rates in three days after Mn addition?P<0.05?,while no significant changes occurred in the average nitrification rates?P>0.05?.Nitrification was best fitted with the first order kinetic model for both soils.However,the zero-order reaction became the best fitted model after MnO₂ was amended,likely due to Mn toxicity to soil nitrifiers?R ²>0.93?,which is indicated by the significant decrease in numbers of amoA gene copies for AOB and AOA?48.2%and 39.1%,respectively??P<0.05?.No significant nitrification occurred in the highly acidic soils and MnO₂addition had a minimal effect?P>0.05?.2.In the subtropical paddy soils with alternate wetting and dry cycles,redox conditions were simulated by adjusting soil moisture content to 50%,100%and 200%of water holding capacity?WHC?,respectively.Results showed that the maximum net rates of soil nitrification(V_a1)decreased significantly with increasing soil moisture content,and so did the average net rates of soil nitrification(V_a2)?P<0.05?.Manganese oxide significantly decreased V_a1?P<0.05?,whereas it had no significant effects on V_a2?P>0.05?.Manganese oxide changed the best fitting nitrification kinetic model from first-to zero-order for both 50 and 200%of WHC treatments,but it had no effect on the treatment of 100%WHC.The maximum emission rates of N₂O significantly increased with increasing soil moisture contents?P<0.05?.Manganese oxide retarded the maximum rate of N₂O emission at 100%WHC,whereas it depressed the rate from 1084 to 225?g N kg^-1 day^-1 at 200%WHC?P<0.05?.The results indicate that the effects of Mn oxide on regulating nitrification and N₂O emissions are associated with water regime-dictated by redox potential.These finding may have important implications in regulating N cycling in the rice-based production ecosystems.3.Weakly and highly acidic soils with high organic matter were collected from the subtropical and temperate forests of China and treated with 0%or 3%MnO₂.Nitrification rates were significantly decreased in the highly acidic soils?P<0.05?,while no significant influences occurred in the weakly acidic soils?P>0.05?.Nitrification was best fitted by the firstorder kinetic model for both types of soils?R²>0.95?.However,the zero-order reaction became the best fitted model when the soils were amended with MnO₂?R²>0.95?.Numbers of AMO amoA gene copy decreased significantly after Mn addition?P<0.05?.Nitrification dynamics were altered by MnO₂ in weakly acidic soils,probably due to Mn toxicity when the soils were treated with 0%or 3%organic matter.The results showed that organic matter significantly decreased the net nitrification rates?P<0.05?,while abundance of ammonia monooxygenase was significantly increased for AOB and AOA?P<0.05?,which implies that organic matter may be another important factor involved in soil nitrification,but the mechanisms of effects may be different from Mn O₂.Base on the above results,sterilized experiments were conducted to study the biotic and abiotic interactions between MnO₂ and N transformation,and their respective contributions.The major conclusions are as follows:4.The amount of NH₄⁺sorbed at equilibrium increased with increasing NH₄⁺concentrations and with increasing p H in the acidic purple soils.The amount of NH₄⁺sorbed onto soil significantly decreased after sterilization,while the addition of MnO₂significantly increased NH₄⁺sorption.Sorption of NH₄⁺in the acidic soils was well described by the Langmuir equation?R²>0.95?,whereas the amount of NO₃^-sorbed by the acidic soils was very small,less than 0.02 mg kg^-1 in the weakly acidic soil and negative sorption occurred in the highly acidic soils.Addition of MnO₂ had no significant influences on NO₃^-sorption by the acidic soils?P>0.05?,implying that MnO₂ has no influence on NO₃^-fixation.5.Microbial assimilation and abiotic fixation resulted in immobilization of inorganic nitrogen in acidic soils.Soil microbial assimilation was the dominated mechanism of NH₄⁺-Nimmobilization.The addition of 3%MnO₂ had no significant effects on either microbial assimilation?13.4%?or abiotic fixation?6.3%?of NH₄⁺-N?P>0.05?.The acidic soils had minimal immobilization for NO₃^--N in terms of both microbial assimilation and abiotic fixation.Addition of MnO₂ significantly decreased microbial assimilation for NO₃^--N from 1.96%to 0.12%?P<0.05?,while abiotic fixation was increased for NO₃^--N from 1.76%to 4.02%?P>0.05?.The mechanism of MnO₂ effects on soil microbial assimilation may be associated with Mn toxicity to soil nitrifiers,while its effect on abiotic fixation of NO₃^--N is unknown.In summary,Mn oxide plays an important role in nitrification in acidic soils and the effects vary with different ecosystems.Manganese oxides retarded nitrification and decreased nitrifier activity under oxic conditions,whereas it enhanced soil nitrification,and decreased emission rate of nitrous oxide under anoxic conditions.The possible mechanisms include a toxic effect of Mn on microorganisms under oxic conditions and the use of Mn as an alternate electron receptor to oxidize NH₃/NH₄⁺to N₂ or compete with NO₃^-as electro receptor,thereby decreasing N₂O emission when O₂ is depleted under anoxic conditions.Furthermore,the addition of MnO₂ had no significant influences on NO₃^-sorption by the acidic soils?P>0.05?,implying that MnO₂ has no influence on NO₃^-fixation,which will not be one of the reasons of nitrification variation in acidic soils.The results provide new insight of pathways for nitrification in acid soil,which may have important implications for improving management of N fertilizer,land use and greenhouse gas control.