摘要：豆科植物在氮素缺乏的荒漠生态系统中大量存在，是该生态系统提供有效氮的中心， 也是这一区域重要的先锋物种。然而，到目前为止，针对荒漠生态系统豆科植物对氮素吸收 利用的研究还鲜见报道。本研究选择古尔班通古特沙漠广泛分布的弯花黄芪（Astragalus flexus）和镰荚黄芪（Astragalus arpilobus）为研究对象，分别在0 ~ 5、5 ~ 15 cm 土层添加 3 种不同形态氮（15N-NH4 +、15N-NO3 -、15N-Glycine），研究两种植物及各器官对不同形态氮 素的吸收、分配策略。结果表明：（1）在不同土层中，两种植物均偏好吸收硝态氮，且弯 花黄芪、镰荚黄芪对硝态氮的最高吸收速率分别为3.26、2.59 μg · g–1 · h–1。（2）在不同土 层中，植物各器官间均对不同氮源吸收及分配有显著性差异（P < 0.05），弯花黄芪根的15N 吸收量均大于镰荚黄芪根的15N 吸收量，且3 种不同形态氮主要分配于叶。（3）在不同土 层中，不同氮源对两种植物的贡献率均为：15N-NO3 - > 15N-Glycine > 15N-NH4 +，且硝态氮对 弯花黄芪氮素吸收的贡献率在37% ~ 41%之间，而对镰荚黄芪氮素吸收的贡献率最高可达 45%。（4）植物各器官间均对不同形态氮的回收率存在显著性差异（P < 0.05）；在0 ~ 5 cm 土层中，植物各器官对硝态氮的回收率均为：叶>茎>根，而5 ~ 15 cm 土层中，弯花黄芪表 现为：叶>根>茎。总体上，在古尔班通古特沙漠生态系统中，不同生活型豆科植物对氮素 吸收及分配能力既有一致性也有差异性，且受到不同土壤深度、氮形态的影响。该结果为新 疆干旱、半干旱区豆科植物的氮吸收利用及分配提供理论依据。
摘要： Atmospherically-deposited nitrogen (N) can stimulate complex soil N metabolisms and accumulations over time. Whether long-term (decadal) N deposition effects on soil N transformations and functional microbes differ from the short-term (annual) effects have rarely been assessed. Here we conducted a laboratory 15N tracing study with soil samples from a short-term (one year) N addition site and a long-term (12 years) site in a subtropical forest. The effects of simulated N deposition on soil N2O emissions, N transformation rates and microbial nitrifying and denitrifying genes were determined. Our results showed that: (1) long-term N addition did not change soil N2O fluxes significantly in comparison to the short-term N addition. Denitrification, heterotrophic nitrification and autotrophic nitrification contributed 53%, 23% and 18% to total N2O emissions, respectively. (2) Autotrophic nitrification was the dominant N transformation process, except for the high-N treatment at the long-term site. The magnitude of soil N transformation rates was significantly different among N addition treatments but not between short- and long-term N addition sites. However, long-term N addition changed the responses of specific N transformation rates to N addition markedly, especially for the rates of nitrification, organic N mineralization to NH4+, NO3- immobilization and dissimilatory NO3- reduction to NH4+ (DNRA). (3) Responses of ammonia oxidizing archaea and bacteria (AOA and AOB) were stronger than denitrifying N2O-producers (nirK) and denitrifying N2O-reducers (nosZ) at the long-term site compared to the short-term site. (4) The close correlations among N2O flux, functional genes and soil properties observed at the short-term site was weakened at the long-term site, posing a decreased risk for N losses in the acid subtropical forest soils. There is evidence for an adaptation of functional microbial communities to the prevailing soil conditions and in response to long-term natural and anthropogenic N depositions.
Brazil is recognized as a prominent renewable energy producer due to the production of ethanol from sugarcane. However, in order for this source of energy to be considered truly sustainable, conservation management practices, such as harvesting the cane green (without burning) and retaining the trash in the field, need to be adopted. This management practice affects mostly the nitrogen (N) cycle through the effect of trash on immobilization–mineralization of N by soil microorganisms. The aim of the experiments reported here was to evaluate N recovery from trash (trash-N) by sugarcane during three ratoon crop seasons: 2007, 2008 and 2009. Two field experiments were carried out, one in Jaboticabal and the other in Pradopolis, in the state of Sao Paulo, Brazil. The experiments were set up in a randomized block design with four replications. Within each plot, microplots were installed where the original trash was replaced by trash labelled with 15N, and maintained up to the fourth crop cycle. Trash-N recovery was higher in the Jaboticabal site, the most productive one, than in the Pradópolis site. The average trash-N recovery across the two sites after three crop cycles was 7.6 kg ha−1 (or 16.2% of the initial N content in trash), with the remaining trash-N being incorporated into soil organic matter reserves. While these results indicate that the value of trash for sugarcane nutrition is limited in the short term, maintaining trash on the field will serve as a long-term source of N and C for the soil.