Soil carbon and nitrogen cycle

Sugarcane has been widely used as a biofuel crop due to its high biological productivity, ease of conversion to ethanol, and its relatively high potential for greenhouse gas reduction and lower environmental impacts relative to other derived biofuels from traditional agronomic crops. In this investigation, we studied four sugarcane cultivars (H-65-7052, H-78-3567, H-86-3792 and H-87-4319) grown on a Hawaiian commercial sugarcane plantation to determine their ability to store and accumulate soil carbon (C) and nitrogen (N) across a 24-month growth cycle on contrasting soil types. The main study objective establish baseline parameters for biofuel production life cycle analyses; sub-objectives included (1) determining which of four main sugarcane cultivars sequestered the most soil C and (2) assessing how soil C sequestration varies among two common Hawaiian soil series (Pulehu-sandy clay loam and Molokai-clay). Soil samples were collected at 20 cm increments to depths of up to 120 cm using hand augers at the three main growth stages (tillering, grand growth, and maturity) from two experimental plots at to observe total carbon (TC), total nitrogen (TN), dissolved organic carbon (DOC) and nitrates (NO−3) using laboratory flash combustion for TC and TN and solution filtering and analysis for DOC and NO−3. Aboveground plant biomass was collected and subsampled to determine lignin and C and N content. This study determined that there was an increase of TC with the advancement of growing stages in the studied four sugarcane cultivars at both soil types (increase in TC of 15–35 kg·m2). Nitrogen accumulation was more variable, and NO−3 (<5 ppm) were insignificant. The C and N accumulation varies in the whole profile based on the ability of the sugarcane cultivar’s roots to explore and grow in the different soil types. For the purpose of storing C in the soil, cultivar H-65-7052 (TC accumulation of ~30 kg·m−2) and H-86-3792 (25 kg·m−2) rather H-78-3567 (15 kg·m−2) and H-87-4319 (20 kg·m−2) appeared to produce more accumulated carbon in both soil types.

Nitrogen is cycled throughout ecosystems by a suite of biogeochemical processes. The high complexity of the nitrogen cycle resides in an intricate interplay between reversible biochemical pathways alternatively and specifically activated in response to diverse environmental cues. Despite aggressive research, how the fundamental nitrogen biochemical processes are assembled and maintained in fluctuating soil redox conditions remains elusive. Here, we address this question using a kinetic modelling approach coupled with dynamical systems theory and microbial genomics. We show that alternative biochemical pathways play a key role in keeping nitrogen conversion and conservation properties invariant in fluctuating environments. Our results indicate that the biochemical network holds inherent adaptive capacity to stabilize ammonium and nitrate availability, and that the bistability in the formation of ammonium is linked to the transient upregulation of the amo-hao mediated nitrification pathway. The bistability is maintained by a pair of complementary subsystems acting as either source or sink type systems in response to soil redox fluctuations. It is further shown how elevated anthropogenic pressure has the potential to break down the stability of the system, altering substantially ammonium and nitrate availability in the soil, with dramatic effects on biodiversity.

Although chemical fertilizers have long been used to meet the high demand of nitrogen (N), the most common limiting nutrient to plant growth, the frequent use of this fertilizer gradually deteriorates soil fertility in addition to its high cost, lower supply and agronomic use efficiency in Ethiopia. Nevertheless, N-fixing cyanobacterial biofertilizers are eco-friendly, and currently considered important to support the developing organic agriculture. Therefore, this study was conducted to evaluate the potential of cyanobacterial biofertilizer for the growth and yield of three commonly growing crops in Ethiopia: maize, kale, and pepper under Alfisol and Andosol, and to investigate the potential contribution of cyanobacteria biofertilizer in selected soil fertility parameters. Three independent factorial experiments were conducted simultaneously in the greenhouse. Each experiment included a factorial combination of four nitrogen fertilizer sources applied at recommendation rate for each crop (control, urea, dried cyanobacteria, and liquid cyanobacteria,) and two soil types with acidic and alkaline pH (Alfisols and Andosols, respectively) arranged in a complete randomized design (CRD) with three replications. Application of dried and liquid cyanobacterial biofertilizer treatments significantly improves the soil N, soil organic carbon (SOC) and available P, Fe and Zn. Cyanobacteria treatments were also found to reduce or maintain the mean soil pH. Accordingly, maximum values of all the vegetative growth attributes of kale, and maize were obtained from the application of two comparable-fertilizer treatments: air-dried cyanobacteria and urea while for pepper crops only dried cyanobacteria. Concentrations of N, P, Zn, and Fe in leaves of kale, pepper, and maize were also significantly increased by air-dried cyanobacterial biofertilizer. Overall, dried cyanobacteria improved the growth and yield of the three crops, and the fertility of the soils. Therefore, the use of dry cyanobacterial biofertilizer could be recommended as a supplementary N source to inorganic fertilizer for kale, pepper and maize production in both study sites.