Soil microbial biomass and activity response to repeated drying-rewetting cycles along a soil fertility gradient modified by long-term fertilization management practices

The effects of repeated drying-rewetting (DRW) cycles on the microbial biomass and activity in soils taken from long-term field experiment plots with different fertilization (FERT) management practice histories were studied. We investigated the hypothesis that soil response to DRW cycles differs with soil fertility gradient modified by FERT management practices. The soils were incubated for 51 days, after exposure to either nine or three DRW cycles, or remaining at constant moisture content (CMC) at field capacity. We found that both DRW and FERT significantly affected soil properties including NH₄-N, NO₃-N, dissolved organic C (DOC), microbial biomass C (Cmic), basal soil respiration rate (BSR), urease activity (URE) and dehydrogenase activity (DHD). Except for NH₄-N and BSR, variation in the properties was largely explained by FERT, followed by DRW, and then their interaction. Irrespective of the soils' FERT treatment, repeated DRW cycles significantly raised the DOC and Cmic levels compared with CMC, and the DRW cycles also resulted in a significant decline in BSR and URE and increase in DHD, probably because the organisms were better-adapted to the drying and rewetting stresses. The variations in soil biological properties caused by DRW cycles showed a significantly negative relationship with the soil organic C content measured prior to the start of the DRW experiments, suggesting that soils with higher fertility are better able to maintain their original biological functions (i.e., have a higher functional stability) in response to DRW cycles.     

Land Use and Precipitation Affect Organic and Microbial Carbon Stocks and the Specific Metabolic Quotient in Soils of Eleven Ecosystems of Mt. Kilimanjaro,Tanzania

Tropical ecosystems are under increasing pressure of land‐use changes, strongly affecting the carbon cycle. Conversion from natural to agri‐cultural ecosystems is often accompanied by a decrease in the stocks of organic and microbial carbon (C_org, C_mic) as well as changes in microbial activity and litter decomposition. Eleven ecosystems along an elevation gradient on the slopes of Mt. Kilimanjaro were used to investigate impacts of land‐use changes on C_org and C_mic stocks as well as the specific metabolic respiration quotient (q_sCO₂) in surface soils. Six natural, two semi‐natural and three intensively used agricultural ecosystems were investigated on an elevation gradient from 950 to 3,880 m asl. To estimate the effects of precipitation, rainfall regimes of 3·6 and 20·0 mm were simulated. C_org stocks were controlled by water availability, temperature and net primary production. Agricultural management resulted in decreases of C_org and C_mic stocks by 38% and 76%, respectively. In addition, agricultural systems were characterized by low C_mic:C_org ratios, indicating a decline in available substrate. Enhanced land‐use intensity leads to increased q_sCO₂ (agricultural > semi‐natural > natural). The traditional homegardens stood out as a sustainable land‐use form with high substrate availability and microbial efficiency. Soil CO₂ efflux and q_sCO2 generally increased with precipitation level. We conclude that soils of Mt. Kilimanjaro's ecosystems are highly sensitive to land‐use changes and are vulnerable to changes in precipitation, especially at low elevations. Even though q_sCO₂ was measured under different water contents, it can be used as an indicator of ecosystem disturbances caused by land‐use and management practices. Copyright © 2015 John Wiley & Sons, Ltd.     

Soil microbial biomass response to woody plant invasion of grassland

Woody plant proliferation in grasslands and savannas has been documented worldwide in recent history. To better understand the consequences of this vegetation change for the C-cycle, we measured soil microbial biomass carbon (C_mic) in remnant grasslands (time 0) and woody plant stands ranging in age from 10 to 130 years in a subtropical ecosystem undergoing succession from grassland to woodlands dominated by N-fixing trees. We also determined the ratio of SMB-C to soil organic carbon (C_mic/C_org) as an indicator of soil organic matter quality or availability, and the metabolic quotient (qCO₂) as a measure of microbial efficiency. Soil organic carbon (C_org) and soil total nitrogen (STN) increased up to 200% in the 0–15 cm depth increment following woody plant invasion of grassland, but changed little at 15–30 cm. C_mic at 0–15 cm increased linearly with time following woody plant encroachment and ranged from 400 mg C kg⁻¹ soil in remnant grasslands up to 600–1000 mg C kg⁻¹ soil in older (>60 years) woody plant stands. C_mic at 15–30 cm also increased linearly with time, ranging from 100 mg C kg⁻¹ soil in remnant grasslands to 400–700 mg C kg⁻¹ soil in older wooded areas. These changes in C_mic in wooded areas were correlated with concurrent changes in stores of C and N in soils, roots, and litter. The C_mic/C_org ratio at 0–15 cm decreased with increasing woody plant stand age from 6% in grasslands to <4% in older woodlands suggesting that woody litter may be less suitable as a microbial substrate compared with grassland litter. In addition, higher qCO₂ values in woodlands (0.8 mg CO₂-C g⁻¹ C_mic h⁻¹) relative to remnant grasslands (0.4 mg CO₂-C g⁻¹ C_mic h⁻¹) indicated that more respiration was required per unit of C_mic in wooded areas than in grasslands. Observed increases in C_org and STN following woody plant encroachment in this ecosystem may be a function of both greater inputs of poor quality C that is relatively resistant to decay, and the decreased ability of soil microbes to decompose this organic matter. We suggest that increases in the size and activity of C_mic following woody plant encroachment may result in: (a) alterations in competitive interactions and successional processes due to changes in nutrient dynamics, (b) enhanced formation and maintenance of soil physical structures that promote C_org sequestration, and/or (c) increased trace gas fluxes that have the potential to influence atmospheric chemistry and the climate system at regional to global scales.