Sugarcane (Saccharum sp. hybrids) crops typically grow for 16-24 months in the subtropics, with nitrogen (N) fertiliser generally applied as a single dose between 150 and 250 kg N ha(-1) early in the season. High N fertiliser application coupled with intense rainfall in the subtropics can lead to nitrate leaching and denitrification events that result in low N fertiliser use efficiency and damage to the environment. We investigated whether the use of a slow-release N fertiliser, polymer coated urea (PCU), may be more agronomically effective than urea as an N fertiliser source, by better matching soil N supply to sugarcane N demand. Multi-rate N fertiliser trials comparing biomass production and N accumulation responses of ratoon sugarcane crops to urea and PCU products were conducted across four commercial sugarcane farms in the Australian subtropics, with N fertiliser applied in a band 100-150 mm below the soil surface 2-12 weeks after the previous cane crop was harvested. At two sites, buried mesh bags containing 90 d and 270 d PCU products were destructively sampled over 12-15 months to assess the N release rate under field conditions. Sugarcane biomass yields were responsive to applied N at two of the four sites (P < 0.05) and crop N accumulation was responsive to N fertiliser application at all four sites (P < 0.1). While the mesh bag study clearly indicated a delayed release of N from the PCU products over time, there was no significant effect of N fertiliser source (urea vs PCU) on crop biomass or N accumulation at any site. The lack of any improvement in agronomic N efficiency with the PCU products is attributed to the presence of active roots in ratoon crops combined with the absence of large rainfall events in the months following N fertiliser application in the seasons of study. Modelling, coupled with an understanding of the N release dynamics of PCU products across a different soils and climatic conditions, is required to develop recommendations for PCU products for sugarcane growers in the region, although further trials across a wider range of seasons may be warranted to verify any modelling predictions.

Studies on the responses of soil organic carbon (SOC) and nitrogen dynamics to Holocene climate and environment in permafrost peatlands and/or wetlands might serve as analogues for future scenarios, and they can help predict the fate of the frozen SOC and nitrogen under a warming climate. To date, little is known about these issues on the Qinghai -Tibet Plateau (QTP). Here, we investigated the accumulations of SOC and nitrogen in a permafrost wetland on the northeastern QTP, and analyzed their links with Holocene climatic and environmental changes. In order to do so, we studied grain size, soil organic matter, SOC, and nitrogen contents, bulk density, geochemical parameters, and the accelerator mass spectrometry (AMS) 14C dating of the 216-cm-deep wetland profile. SOC and nitrogen contents revealed a general uptrend over last 7300 years. SOC stocks for depths of 0-100 and 0-200 cm were 50.1 and 79.0 kgC m-2, respectively, and nitrogen stocks for the same depths were 4.3 and 6.6 kgN m-2, respectively. Overall, a cooling and drying trend for regional climate over last 7300 years was inferred from the declining chemical weathering and humidity index. Meanwhile, SOC and nitrogen accumulated rapidly in 1110-720 BP, while apparent accumulation rates of SOC and nitrogen were much lower during the other periods of the last 7300 years. Consequently, we proposed a probable conceptual framework for the concordant development of syngenetic permafrost and SOC and nitrogen accumulations in alpine permafrost wetlands. This indicates that, apart from controls of climate, non-climate environmental factors, such as dust deposition and site hydrology, matter to SOC and nitrogen accumulations in permafrost wetlands. We emphasized that environmental changes driven by climate change have important impacts on SOC and nitrogen accumulations in alpine permafrost wetlands. This study could provide data support for regional and global estimates of SOC and nitrogen pools and for global models on carbon -climate interactions that take into account of alpine permafrost wetlands on the northeastern QTP at mid-latitudes.

Northern peatlands sequester carbon (C) and nitrogen (N) over millennia, at variable rates that depend on climate, environmental variables and anthropogenic activity. The ombrotrophic peatlands of central and northern Alberta (Canada) have developed under variable climate conditions during the last hundreds to thousands of years, while in the course of the twentieth century, some regions were also likely subjected to anthropogenic disturbance. We aimed to quantify peat C and N accumulation rates for the last millennium from seven peatlands to estimate the relative influence of climate and anthropogenic disturbance on C accumulation dynamics. Peatlands have accumulated C at an average rate of 25.3 g C m(-2) year(-1) over the last millennium. Overall, climate was likely a major factor as, on average, highest apparent rates of C accumulation were found around 1100 CE, during the warmer Medieval Climate Anomaly, with lowest rates during the Little Ice Age, around 1750 CE. Local factors, such as disturbance, played a role in C sequestration at the site scale. The average N accumulation rate was 0.55 g N m(-2) year(-1), with high inter- and intra-site variability. In general, N accumulation mirrored patterns in C sequestration for peat deposited pre-1850 CE. However, higher N accumulation rates observed after 1850 CE, averaging 0.94 g N m(-2) year(-1), were not correlated with C accumulation. Moreover, some of the historically strongly accumulating sites may have become less efficient in sequestering C, and vice versa. All seven sites showed a marked decrease in delta N-15 when comparing pre- and post-1850 timeframes, consistent with increasing post-1850 N additions from an atmospheric source, likely biological N fixation. Overall, N was not a driving factor for C accumulation.