A history of extreme disturbance affects the relationship between the abundances of nitrifiers in soil

To understand how and to what extent single or multiple perturbations can alter the relationships between the abundances of different nitrifier groups and nitrification, soil microcosms were exposed to six disturbance treatments: a heat shock, cold shock, or control conditions applied to undisturbed soils or to soils that had previously been subjected to a first heat shock. We monitored the recovery of the abundances of four main nitrifier groups (ammonia-oxidizing archaea and bacteria, AOA and AOB, respectively, andNitrobacterandNitrospiranitrite oxidizers) as well as nitrification activity for 25 days. AOA were sensitive to cold shocks, whereas AOB were not; the latter were sensitive to heat shock. Despite the variations, both groups were resilient to the first disturbance. In contrast,Nitrobacterwas affected by both disturbances, whereasNitrospirawas resistant to both shocks. Prior exposure to a heat shock affected each group's responses as well as the relationships between them. For example, AOB were more vulnerable to heat shock in pre-exposed soils, whereas under the same circumstances, AOA were resilient. Nitrification activity was resistant to the first disturbances, but a legacy effect was observed, and nitrification was highest in Heat-Heat and lowest in Heat-Cold treatments. Overall, our study shows that within soil nitrifiers, temporal patterns and legacy effects interact to result in complex disturbance responses

Adaptation of soil nitrifiers to very low nitrogen level jeopardizes the efficiency of chemical fertilization in west african moist savannas

The moist savanna zone covers 0.5 x 10(6) km(2) in West Africa and is characterized by very low soil N levels limiting primary production, but the ecology of nitrifiers in these (agro) ecosystems is largely unknown. We compared the effects of six agricultural practices on nitrifier activity, abundance and diversity at nine sites in central Ivory Coast. Treatments, including repeated fertilization with ammonium and urea, had no effect on nitrification and crop N status after 3 to 5 crop cycles. Nitrification was actually higher at low than medium ammonium level. The nitrifying community was always dominated by ammonia oxidizing archaea and Nitrospira. However, the abundances of ammonia oxidizing bacteria, AOB, and Nitrobacter increased with fertilization after 5 crop cycles. Several AOB populations, some affiliated to Nitrosospira strains with urease activity or adapted to fluctuating ammonium levels, emerged in fertilized plots, which was correlated to nitrifying community ability to benefit from fertilization. In these soils, dominant nitrifiers adapted to very low ammonium levels have to be replaced by high-N nitrifiers before fertilization can stimulate nitrification. Our results show that the delay required for this replacement is much longer than ever observed for other terrestrial ecosystems, i.e. >5 crop cycles, and demonstrate for the first time that nitrifier characteristics jeopardize the efficiency of fertilization in moist savanna soils

Different groups of nitrite-reducers and N2O-reducers have distinct ecological niches and functional roles in West African cultivated soils

Increasing attention has been paid to microorganisms able to produce nitrous oxide (N2O), a potent greenhouse gas, or reduce it to harmless N-2. Based on previous studies, niche differentiation could exist between nirK- and nirS-nitrite reducers and nosZI- and nosZII-N2O reducers, and nosZII-bacteria would have a key role for N2O reduction in soils. Most previous studies have been performed for agricultural systems but never in the moist savanna zone which covers half a million lane in West Africa and whose soils are among the poorest in nitrogen (N) on earth. Here, we quantified potential gross and net N2O production rates along with the abundances of nirK-, nirS-, nosZl- and nosZII-harbouring bacteria for soils under six agricultural practices with maize rotations (slash-and-burn, chemical fertilization, mulching with or without inclusion of crop legumes, and without any input) after 4 and 5 crop cycles at nine sites in Ivory Coast. Sites and practices influenced denitrifier abundances and activities, the ratio of total abundances of nitrite-to-N2O reducers being highest and lowest for the mulching + green soya and slash-and-burn practices, respectively. Using structural equation modelling, we showed that nirS- and nosZI-bacteria both strongly depended on nitrate availability whereas nirK- and nosZII-bacteria were related to soil organic carbon and pH. Furthermore, potential gross and net N2O production rates depended strongly and only on the abundances of nirS- and nosZI-bacteria. Our results support the view of a clear niche differentiation between these four microbial groups but invalidate the assumption of a prominent functional role of soil nosZII-N2O reducers

Response of microbial functional groups involved in soil N cycle to N, P and NP fertilization in Tibetan alpine meadows

The nitrogen (N) cycle is an important part of earth's biogeochemical cycles and N is a critical element for all life. Whereas the response to N - and more rarely phosphorus, P - fertilization of some microbial groups involved in soil N cycling has been studied, a comprehensive view of how the major microbial groups involved in soil N dynamics respond to combined N and P fertilization is lacking, which restricts our understanding of ecosystem responses to fertilization. Here we investigated the effects of different N, P and NP fertilizer levels (4 N levels without P; 4 P levels without N; and 4 P levels with constant N addition) on the abundances of 9 microbial groups involved in N dynamics. Real time PCR was used to target free N-2 fixers, nitrifiers (bacterial and archaea ammonia oxidizers, AOB and AOA, respectively; and the nitrite oxidizers Nitrobacter and Nitrospira), nitrate reducers, nirK- and nirS-nitrite reducers, and nitrous oxide reducers. Soil physical-chemical characteristics and potential nitrification, PNR, were also measured. N fertilization increased the abundances of AOB and Nitrobacter but did not, affect the abundances of the other groups. P fertilization decreased the abundances of N2 fixers, nitrate reducers and AOA, and increased the abundances of Nitrobacter and nitrous oxide reducers. NP fertilization decreased the abundances of AOA and nirK-nitrite reducers. Using a correlation network analysis, we demonstrate the strong coupling generally observed in these grasslands between N2 fixers, AOA, Nitrospira, narG-nitrate reducers and nirK-denitrifiers (most of them responding to N/P availability, and being known to be favored by low oxygen availability); and between AOB and Nitrobacter (known to be favored by high oxygen and high N levels) that controlled changes in PNR. The observed (de)coupling between the responses of the different microbial groups may have major consequences for N cycling and N losses from fertilized Tibetan alpine meadows