Fires shape the biogeochemistry and functioning of many ecosystems, and fire frequencies are changing across much of the globe. Frequent fires can change soil carbon (C) and nitrogen (N) storage by altering the quantity and chemistry of plant inputs through changes in plant biomass and composition as well as the decomposition of soil organic matter. How decomposition rates change with shifting inputs remains uncertain because most studies focus on the effects of single fires, where transient responses may not reflect responses to decadal changes in burning frequencies. Here, we sampled seven sites exposed to different fire frequencies. In four of the sites, we intensively sampled both soils and plant communities across four ecosystems in North America and Africa spanning tropical savanna, temperate coniferous savanna, temperate broadleaf savanna, and temperate coniferous forest ecosystems. Each site contained multiple plots burned frequently for 33–61 years and nearby plots that had remained unburned over the same period replicated at the landscape scale. Across all sites, repeatedly burned plots had 25–185% lower bulk soil C and N concentrations but also 2–10‐fold lower potential decomposition of organic matter compared to unburned sites. Soil C and N concentrations and extracellular enzyme activities declined with frequent fire because fire reduced both plant biomass inputs into soils and dampened the localized enrichment effect of tree canopies. Examination of soil extracellular enzyme activities revealed that fire decreased the potential turnover of organic matter in the forms of cellulose, starch, and chitin (P < 0.0001) but not polyphenol and lignin (P = 0.09), suggesting a shift in soil C and N cycling. Inclusion of δ¹³C data from three additional savanna sites (19–60 years of altered fire frequencies) showed that soil C losses were largest in sites where estimated tree inputs into soils declined the most (r² = 0.91, P < 0.01). In conclusion, repeated burning reduced C and N storage, consistent with previous studies, but fire also reduced potential decomposition, likely contributing to slower C and N cycling. Trees were important in shaping soil C and N responses across sites, but the magnitude of tree effects differed and depended on how tree biomass inputs into soil responded to fire.

中文翻译：

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通过改变整个生态系统中的植物投入和土壤分解，反复发生的火灾改变了碳和氮的循环

火灾影响着许多生态系统的生物地球化学和功能，并且火灾频率在全球大部分地区都在变化。频繁的大火可以通过改变植物生物量和组成以及土壤有机质的分解来改变植物投入的数量和化学性质，从而改变土壤碳（C）和氮（N）的储存量。分解速率如何随着输入的变化而变化仍然是不确定的，因为大多数研究都集中在单次火灾的影响上，在这种情况下，瞬态响应可能无法反映对燃烧频率的十年变化的响应。在这里，我们抽样了七个暴露于不同火灾频率的地点。在这四个地点中，我们对北美洲和非洲的四个生态系统中的土壤和植物群落进行了密集采样，范围包括热带稀树草原，温带针叶稀树草原，温带阔叶稀树草原，和温带针叶林生态系统。每个地点都包含多处频繁燃烧33-61年的地块，并且在同一时期内未燃烧的附近地块以景观尺度进行了复制。在所有地点，重复焚烧的地块土壤中的碳和氮含量降低了25–185％，但与未焚烧地点相比，有机物的潜在分解量降低了2-10倍。随着火灾的频繁发生，土壤碳和氮的浓度以及细胞外酶的活性下降，这是因为火灾减少了植物向土壤中输入的生物量并削弱了树冠的局部富集作用。对土壤细胞外酶活性的检查表明，火降低了纤维素，淀粉和几丁质形式的有机物的潜在转化率（每个地点都包含多处频繁燃烧33-61年的地块，并且在同一时期内未燃烧的附近地块以景观尺度进行了复制。在所有地点，反复燃烧的地块的土壤碳和氮含量降低了25–185％，但与未燃烧的地点相比，有机物的潜在分解量降低了2-10倍。随着火灾的频繁发生，土壤碳和氮的浓度以及细胞外酶的活性下降，这是因为火灾减少了植物生物量向土壤中的输入并削弱了树冠的局部富集作用。对土壤细胞外酶活性的检查表明，火降低了纤维素，淀粉和几丁质形式的有机物的潜在转化率（每个地点都包含多处频繁燃烧33-61年的地块，并且在同一时期内未燃烧的附近地块以景观尺度进行了复制。在所有地点，反复燃烧的地块的土壤碳和氮含量降低了25–185％，但与未燃烧的地点相比，有机物的潜在分解量降低了2-10倍。随着火灾的频繁发生，土壤碳和氮的浓度以及细胞外酶的活性下降，这是因为火灾减少了植物生物量向土壤中的输入并削弱了树冠的局部富集作用。对土壤细胞外酶活性的检查表明，火降低了纤维素，淀粉和几丁质形式的有机物的潜在转化率（与未燃烧的地块相比，反复燃烧的地块的土壤碳和氮浓度降低了25–185％，但有机物的潜在分解量也降低了2-10倍。随着火灾的频繁发生，土壤碳和氮的浓度以及细胞外酶的活性下降，这是因为火灾减少了植物生物量向土壤中的输入并削弱了树冠的局部富集作用。对土壤细胞外酶活性的检查表明，火降低了纤维素，淀粉和几丁质形式的有机物的潜在转化率（与未燃烧的地块相比，反复燃烧的地块的土壤碳和氮浓度降低了25–185％，但有机物的潜在分解量也降低了2-10倍。随着火灾的频繁发生，土壤碳和氮的浓度以及细胞外酶的活性下降，这是因为火灾减少了植物生物量向土壤中的输入并削弱了树冠的局部富集作用。对土壤细胞外酶活性的检查表明，火降低了纤维素，淀粉和几丁质形式的有机物的潜在转化率（P  <0.0001），而不是多酚和木质素（P  = 0.09），表明土壤C和N循环发生了变化。δ的夹杂^13个从三个附加稀树草原位点（19-60岁改变火频率）C数据表明：土壤碳损失中的网站，其中估计树投入土壤下降最多（分别为最大- [R ²  = 0.91，P  <0.01）。总之，重复燃烧减少了碳和氮的存储，与先前的研究一致，但是燃烧还减少了潜在的分解，可能导致碳和氮循环变慢。树木对于塑造不同地点的土壤碳和氮响应非常重要，但是树木的影响程度不同，并取决于树木向土壤中输入的生物量对火的反应方式。