The relative contribution of different microbial groups to soil organic matter (SOM) turnover and utilisation of rhizodeposits during a cropping season has remained largely unknown. We used a long-term field experiment (started in 1956), in which C3 crops were replaced with C4 silage maize in 2000, to investigate dynamics of fungi and bacterial groups and their utilisation of ‘young-C4’ and ‘old-C3’ SOM-derived resource every second week during the cropping season (June–Oct). Treatments include bare fallow, unfertilised, fertilised with mineral N and fertilised with farmyard manure (FYM) addition. Extracted soil phospholipid fatty acids (PLFAs) were pooled into Gram-positive, Gram-negative bacteria and fungi (18:2ω6,9) groups and their δ 13 C values determined. Total PLFAs amount correlated to the SOM contents (highest in FYM) and increased over the cropping season in N-fertilised and FYM treatments. As a result of a peak in plant growth during a period with frequent rain events in August, δ 13 C of total PLFAs significantly increased from − 23.8 to − 21.6‰ and − 26.1 to − 24.7‰, in N-fertilised and FYM addition, respectively. This clearly indicated a shift in microbial utilisation from old to young SOM sources, which was linked to increased soil moisture contents and fungal biomass. The abundance of Gram-positive increased and that of Gram-negative bacteria decreased until August and vice versa thereafter. The mean δ 13 C values of individual microbial groups were highest in fungi (corresponding to their seasonal biomass variation) followed by Gram-positive and Gram-negative bacteria. The results clearly demonstrated that irrespective of fertilisation type, fungi were the main players in seasonal SOM dynamics and were strongly influenced by soil moisture and phenological stage of the maize (i.e. rhizodeposition). Disentangling these microbial controls on C resources utilisation will be crucial for understanding C cycling during a cropping season or on an ecosystem scale.

中文翻译：

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长期试验中玉米生长季真菌和细菌群及其碳源的动态变化

在作物季节期间，不同微生物群对土壤有机质 (SOM) 周转和根际沉积物利用的相对贡献在很大程度上仍然未知。我们使用长期田间试验（始于 1956 年），其中 C3 作物于 2000 年被 C4 青贮玉米替代，以研究真菌和细菌群的动态及其对“年轻 C4”和“老 C3”的利用在种植季节（6 月至 10 月）期间，每两周一次 SOM 衍生资源。处理方法包括裸休、未施肥、用矿物质 N 施肥和添加农家肥 (FYM) 施肥。提取的土壤磷脂脂肪酸 (PLFA) 被合并到革兰氏阳性、革兰氏阴性细菌和真菌 (18:2ω6,9) 组中，并确定它们的 δ 13 C 值。PLFA 总量与 SOM 含量相关（在 FYM 中最高），并且在施氮和 FYM 处理的作物季节增加。由于在 8 月频繁降雨事件期间植物生长达到峰值，在施氮和 FYM 添加中，总 PLFAs 的 δ 13 C 从 - 23.8 到 - 21.6 ‰ 和 - 26.1 到 - 24.7 ‰ 显着增加，分别。这清楚地表明微生物利用从古老的 SOM 来源转向年轻的 SOM 来源，这与土壤水分含量和真菌生物量的增加有关。直到八月，革兰氏阳性菌的丰度增加，革兰氏阴性菌的丰度减少，反之亦然。单个微生物群的平均δ 13 C 值在真菌中最高（对应于它们的季节性生物量变化），其次是革兰氏阳性菌和革兰氏阴性菌。结果清楚地表明，无论施肥类型如何，真菌都是季节性 SOM 动态的主要参与者，并且受到土壤水分和玉米物候阶段（即根际沉积）的强烈影响。解开这些微生物对碳资源利用的控制对于理解作物季节或生态系统范围内的碳循环至关重要。