Management intensity controls soil N2O fluxes in an Afromontane ecosystem

Abstract

Studies that quantify nitrous oxide (NO) fluxes from African tropical forests and adjacent managed land uses are scarce. The expansion of smallholder agriculture and commercial agriculture into the Mau forest, the largest montane forest in Kenya, has caused large-scale land use change over the last decades. We measured annual soil NO fluxes between August 2015 and July 2016 from natural forests and compared them to the NO fluxes from land either managed by smallholder farmers for grazing and tea production, or commercial tea and eucalyptus plantations (n=18). Air samples from 5 pooled static chambers were collected between 8:00am and 11:30am and used within each plot to calculate the gas flux rates. Annual soil NO fluxes ranged between 0.2 and 2.9kgNhayr at smallholder sites and 0.6-1.7kgNhayr at the commercial agriculture sites, with no difference between land uses (p=0.98 and p=0.18, respectively). There was marked variation within land uses and, in particular, within those managed by smallholder farmers where management was also highly variable. Plots receiving fertilizer applications and those with high densities of livestock showed the highest NO fluxes (1.6±0.3kgNO-Nhayr, n=7) followed by natural forests (1.1±0.1kgNO-Nhayr, n=6); although these were not significantly different (p=0.19). Significantly lower fluxes (0.5±0.1kgNhayr, p<0.01, n=5) were found on plots that received little or no inputs. Daily soil NO flux rates were not correlated with concurrent measurements of water filled pore space (WFPS), soil temperature or inorganic nitrogen (IN) concentrations. However, IN intensity, a measure of exposure of soil microbes (in both time and magnitude) to IN concentrations was strongly correlated with annual soil NO fluxes.

Soil carbon dioxide and methane fluxes from forests and other land use types in an African tropical montane region

“…The study area was located in the southwest Mau Forest in Western Kenya at approximately 2200 m above sea level. During the study period (August 2015 to August 2016) annual rainfall was 2050 mm, while the mean annual air temperature was 16.6 ± 3.9°C (Wanyama et al 2018). The rainfall pattern at the study site is bimodal.…”

“…At the smallholder site, forest (SHF1, SHF2, and SHF3), grazing land (SHG1, SHG2 and SHG3) and tea plantations (SHT1, SHT2 and SHT3) were the major land use types. Detailed information was reported by Wanyama et al (2018) and is also presented in Table 7 in Appendix. The landscape of the study site is undulating and can be divided into lower, mid and crest slope positions: the forest sites were on one side of a valley while the converted land use types were on the other side.…”

“…In contrast, plot SHG3 consisted of a large area (39 ha) where 41 cattle grazed continuously through the year. Further information on plot history is reported by Wanyama et al (2018).…”

“…and NO 3 was calculated by integrating the area under respective curves and herein referred to as NH 4 ? -N intensity and NO 3 --N intensity, respectively (Burton et al 2008;Wanyama et al 2018). , and quantity and timing of fertilization (kg N ha -1 ) applied to two tea plots (SHT1 and SH2), water filled pore space (%WFPS) and precipitation (grey bars) (c) and soil temperature (d) from forest, grazing and tea land use types at the smallholder site (SH) in the South West Mau forest area, Kenya…”

Land-use change and Biogeochemical controls of soil CO2, N2O and CH4 fluxes in Cameroonian forest landscapes

“…Most conversion of forests to agriculture occurs in the tropics, and while datasets from this region have increased over the past several decades, there are still large data gaps and especially gaps in our understanding that limit our ability to constrain non-CO 2 GHG budgets. There have been a number of studies that have quantified the effects of land-use change on gas fluxes from Asia and Latin America (Keller et al 1993;Verchot et al 1999Verchot et al , 2000Verchot et al , 2008Erickson et al 2001;Veldkamp et al 2008Veldkamp et al , 2013Aini et al 2015;van Lent et al 2019), but only a few studies have been conducted in Africa (Arias-Navarro et al 2017;Wanyama et al 2018Wanyama et al , 2019. In this study, we report new data on GHG emissions from various typical land-uses from a Congolean rainforest landscape in Cameroon, where swidden agriculture is being replaced by cacao agroforestry.…”