Nitrogen availability determines the long-term impact of land-use change on soil carbon stocks in Ghana

Enhancing the capacity of agricultural soils to resist soil degradation and to mitigate climate change requires long-term assessments of land-use systems. Such long-term evaluations, particularly regarding low-input livestock systems, are limited. The location of the current study, which is a research exhibition farm cultivated and managed (harvested and weeded to maintain plot purity) since 50 years, with surrounding native and grazed natural grasslands under identical climatic and environmental conditions. Moreover, the location offers unique opportunities to test the long-term effects of converting native grassland to agricultural land on SOC storage. These resources are particularly valuable as long-term experiments designed for this purpose are absent in the sub-Saharan Africa region. The main aim of this study was to assess the impact of long-term land-use practices on soil carbon storage as well as the potential role of tannin-rich forages and soil nutrient status.<br>The study was conducted at and around a research exhibition farm of the Council for Scientific and Industrial Research (CSIR), Ghana, located at 5°70´N, 0°29´W and 49 m a.s.l. The surface lithology is of non-carbonate sedimentary with coarse sandy loam soils belonging to the haplic acrisol group (IUSS, 2014). The mean monthly temperature ranged from 21 to 31°C and a monthly rainfall of 13–205 mm (annual rainfall is approximately 800 mm). The major rainy season is from April to mid-July with the minor rainy season in October. Thus, the climate is moist semi-arid with a growing period lasting between 120 – 180 days (Ghana Meteorological Services Department, 2018). The land cover is a native tropical grassland Savannah with very limited disturbance. Parts of this native grassland have been converted to different agricultural uses since 1966. <br>Briefly, the land-use types comprise arable field crops and grazed-seeded grasslands (both at field scales with at least 1 ha), as well as the above mentioned experimental research exhibition farm. The research exhibition farm comprised 59 species with relevance for forage production, representing four plant functional groups: cut-use grasses (38 species), cut-use legume herbs (11 species), cut-use legume trees and shrubs (7 species), and cut-use non-legume trees and shrubs (3 species) at plot scales (25 – 30 m²). Each plot was harvested once every year and weeded to maintain species purity, with no other management being conducted. Individual species of each functional group constituted a replicate. Similarly, the different fields under each land-use type constituted a replicate. The sampling procedure adopted in this study followed recommendations by Saiz and Albrechts (2016). For each sampling plot or field, four replicates of soil samples were taken (0–30 cm depth). Replicate samples obtained were bulked, thoroughly mixed, and a representative sample taken into a zipped polythene bag for analyses. For the estimation of soil bulk density, four sets of soil samples were taken from each experimental unit (plots/fields) using a stainless-steel core sampler at depths of 0 – 5 cm, 5 – 10 cm, 10 – 15 cm, 15 – 20 cm and 20–25 cm. Soil samples meant for C and nutrient analysis were initially oven-dried at 30ºC for 48 hours. Dried samples were sieved with a 2 mm sieve to remove coarse particles and plant roots. Sieved samples were milled and stored in a desiccator before analyses. <br>Soil samples were analyzed for C and total soil N (TN) using the C/N analyser (Vario Max CN, Germany), using aspirin (50 mg; N = 9.7 %; C = 34.0 %) and a standard soil sample (1 g; N = 1.2 %; C = 1.4 %) after every 10 test soil samples to aid calibration of the equipment. Bulk density was estimated after oven drying at 105ºC. It was assumed that soil samples did not contain inorganic C because pH values were less than seven and because no liming or any other amendment was carried out during the past 50 years; therefore, total C was considered as SOC. Additional randomly performed HCl tests confirmed this assumption. Soil pH was determined according to methods by Wiesmeier et al. (2012). Soil pH was measured directly with a pH meter (Microprocessor pH/ION Meter, PMX 3000, WTW) after adding 0.0125 M CaCl₂ solution to each sample in the ratio of 1:2.5 (soil: CaCl2 solution). Plant available phosphorus (P) and exchangeable potassium (K) were extracted from 1 g air-dried fine soil (&lt;2 mm) using Bray 2 solution, the reagents being 0.1 M HCl and 0.03 M NH4F (Bray and Kurtz, 1945). K was determined using flame-photometry, and P measured calorimetrically at 882 nm (Miller and Arai, 2016) after reaction with ammonium molybdate and development of the ‘Molybdenum (Mo) Blue’ colour (within 30 min.). <br>Plant samples for CT determination were taken from soil sampling sites in January 2018 during the late annual growth stage. Sampling included leaves and leaf stalks from dicots, which consisted of legume herbs, legume and non-legume tree and shrub species. After cutting, samples were immediately cooled on ice before being freeze-dried, milled with a ball mill and stored in a freezer at -28°C until further analyses. Condensed tannins (CT, syn. proanthocyanidins), which consisted of extractable CTs (ECT), protein-bound tannins (PCT) and fibre-bound tannins (FCT), were determined according to methods prescribed by Terrill et al. (1992). The CTs were extracted from 20 mg plant samples using an acetone/water mixture (80/20 v/v), and their concentrations determined using a spectrophotometer (Libra S22, Biochrom) at 550 nm. Total CT (TCT) was calculated as the sum of ECT, PCT and FCT for each candidate species. Soil bulk density and C stock were estimated according to methods by Guo and Gifford (2002). Change in soil C stock as a result of the land-use change was estimated as the difference between soil C stock of native vegetation (natural baseline) and the land use type under consideration (FAO, 2019). Annual changes in soil C (deltaSOC) stock were estimated by dividing each difference by the number of years (50 years) since the native grassland was converted. <br><br>