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We investigate the interactions between natural resource-based poverty traps and food security for smallholder farms in highland Kenya using a recently developed system dynamics bio-economic model. This approach permits examination of the complex interactions and feedback between farm-household economic decision-making and long term soil fertility dynamics that appear to characterize persistent poverty and food insecurity among smallholders in rural highland Kenya. We examine the effects of changing initial endowments of land and stocks of on-farm soil organic matter on smallholders’ well-being, as reflected in several different indicators. We show that larger and higher quality land endowments permit accumulation of cash and livestock resources and conservation of soil organic matter relative to smaller or more degraded farms. This suggests the existence of asset thresholds that divide food secure households from insecure ones.

Loss of agroecosystem soil functions due to soil quality (SQ) degradation impacts Africa's agricultural viability and food security. Primary forest and farm fields deforested between 1930 and 2000 were sampled along a chronosequence on two parent materials in western Kenya. Two traditional long-term management systems were sampled: continuous low-input maize (Zea mays; Co), and kitchen garden (Ki) polyculture with organic inputs. Physical, biological, and chemical SQ indicators were measured. Degradation in Co followed exponential decay trends for most indicators (organic matter, active C, water-stable aggregates, available water capacity, electrical conductivity, CEC, pH, Ca, Mg and Zn), as well as for yield. Organic matter quality declined linearly, suggesting degradation will continue. For both parent materials and most indicators degradation of 25–93% below initial values resulted, but with ≤40% further drop below initial values and for more indicators under Co than Ki. P, Zn and possibly K accumulated over time under Ki. The extent of degradation was influenced by parent material. In conclusion, a basic accessible set of SQ indicators was successfully used to describe soil degradation dynamics under cultivation. Results suggest that regular organic inputs can significantly reduce degradation, especially of nutrient retention and soil structure, after forest conversion.

Soil degradation is one of the most serious threats to sustainable crop production in many tropical agroecosystems where extensification rather than intensification of agriculture has occurred. In the highlands of western Kenya, we investigated soil nitrogen (N) and phosphorus (P) constraints to maize productivity across a cultivation chronosequence in which land-use history ranged from recent conversion from primary forest to 100 years in continuous cropping. Nutrient treatments included a range of N and P fertilizer rates applied separately and in combination. Maize productivity without fertilizer was used as a proxy measure for indigenous soil fertility (ISF). Soil pools of mineral nitrogen, strongly bound P and plant-available P decreased by 82%, 31% and 36%, and P adsorption capacity increased by 51% after 100 years of continuous cultivation. For the long rainy season (LR), grain yield without fertilizer declined rapidly as cultivation age increased from 0 to 25 years and then gradually declined to a yield of 1.6Mgha � 1 , which was maintained as time under cultivation increased from 60 to 100 years. LR grain yield in the old conversions was only 24% of the average young conversion grain yield (6.4Mgha � 1 ). Application of either N or P alone significantly increased grain yield in both the LR and short rainy (SR) seasons, but only application of 120kgNha � 1 on the old conversion increased yield by 41Mgha � 1 . In both SR and LR, there was a greater average yield increment response to N and P when applied together (ranging from 1 to 3.8Mgha � 1 for the LR), with the greatest responses on the old conversions. The benefit‐cost ratio (BCR) for applying 120kgNha � 1 alone was o1 except on the old conversions, while BCRs were41 for applying 25kgPha � 1 alone at all levels of conversion for both seasons. Application of both N (120kgNha � 1 ) and P (25kgPha � 1 ) on the old conversions resulted in the greatest BCRs. This study clearly indicates that maize productivity responses to N and P fertilizer are significantly affected by the age of cultivation and its influence on ISF, but that loss of productivity can be restored rapidly when these limiting nutrients are applied. Management strategies should consider ISF and economic factors to determine optimal N and P input requirements for achieving and sustaining profitable crop production on degraded soils.

Anthropogenic perturbations have profoundly modified the Earth’s biogeochemical cycles, the most prominent of these changes being manifested by global carbon (C) cycling. We investigated long-term effects of human-induced land-use and land-cover changes from native tropical forest (Kenya) and subtropical grassland (South Africa) ecosystems to agriculture on the dynamics and structural composition of soil organic C (SOC) using elemental analysis and integrated 13 C nuclear magnetic resonance (NMR), near-edge X-ray absorption fine structure (NEXAFS) and synchrotron-based Fourier transform infrared-attenuated total reflectance (Sr-FTIR-ATR) spectroscopy. Anthropogenic interventions led to the depletion of 76%, 86% and 67% of the total SOC; and 77%, 85% and 66% of the N concentrations from the surface soils of Nandi, Kakamega and the South African sites, respectively, over a period of up to 100 years. Significant proportions of the total SOC (46‐73%) and N (37‐73%) losses occurred during the first 4 years of conversion indicating that these forest- and grassland-derived soils contain large amounts of labile soil organic matter (SOM), potentially vulnerable to degradation upon human-induced land-use and land-cover changes. Anthropogenic perturbations altered not only the C sink capacity of these soils, but also the functional group composition and dynamics of SOC with time, rendering structural composition of the resultant organic matter in the agricultural soils to be considerably different from the SOM under natural forest and grassland ecosystems. These molecular level compositional changes were manifested: (i) by the continued degradation of O-alkyl and acetal-C structures found in carbohydrate and holocellulose biomolecules, some labile aliphatic-C functionalities, (ii) by side-chain oxidation of phenylpropane units of lignin and (iii) by the continued aromatization and aliphatization of the humic fractions possibly through selective accumulation of recalcitrant H and C substituted aryl-C and aliphatic-C components such as (poly)-methylene units, respectively. These changes appeared as early as the fourth year after transition, and their intensity increased with duration of cultivation until a new quasi-equilibrium of SOC was approached at about 20 years after conversion. However, subtle but persistent changes in molecular structures of the resultant SOM continued long after (up to 100 years) a steady state for SOC was approached. These molecular level changes in the inherent structural composition of SOC may exert considerable influence on biogeochemical cycling of C and bioavailability of essential nutrients present in association with SOM, and may significantly affect the sustainability of agriculture as well as potentials of the soils to sequester C in these tropical and subtropical highland agroecosystems.

The soil environment is a primary component of the global biogeochemical sulfur (S) cycle, acting as a source and sink of various S species and mediating oxidation state changes. However, ecological significance of the various S forms and the impacts of human intervention and climate on the amount and structural composition of these compounds are still poorly understood. We investigated the long-term influences of anthropogenically mediated transitions from natural to managed ecosystems on molecular-level speciation, biogeochemical dynamics, and the apparent temperature sensitivity of S moieties in temperate, subtropical, and tropical environments with mean annual temperature (MAT) ranging from 5 degrees C to 21 degrees C, using elemental analysis and X-ray absorption near-edge structure (XANES) spectroscopy. Land-use and land-cover changes led to the depletion of total soil S in all three ecoregions over a period of up to 103 years. The largest decline occurred from tropical forest agroecosystems (67% Kakamega and 76% Nandi, Kenya), compared to losses from temperate (36% at Lethbridge, Canada, and 40% at Pendleton, USA) and subtropical (48% at South Africa) grassland agroecosystems. The total S losses correlated significantly with MAT. Anthropogenic interventions profoundly altered the molecular-level composition and resulted in an apparent shift in oxidation states of organic S from native ecosystems composed primarily of S moieties in intermediate and highly reduced oxidation states toward managed agroecosystems dominated by organic S rich in strongly oxidized functionalities. The most prominent change occurred in thiols and sulfides, the proportion of which decreased by 46% (Lethbridge) and 57% (Pendleton) in temperate agroecosystems, by 46% in subtropical agroecosystems, and by 79% (Nandi) and 81% (Kakamega) in tropical agroecosystems. The proportion of organic S directly linked to O increased by 81%, 168%, 40%, 92%, and 85%, respectively. Among the various organic S functionalities, thiols and sulfides seem to have higher apparent temperature sensitivity, and thus these organic S moieties may become prone to losses due to land-use changes, even from the cooler regions of the world if MAT of these regions rise in the future.

We investigate natural resource-based poverty traps using a system dynamics model of smallholder farms in highland Kenya. System dynamics modeling is well-suited to examining the complex interactions and feedback between farm-household economic decision making and long-term soil dynamics which may be at the source of persistent poverty among smallholders in this region. We examine the effects of changing initial endowments of land, labour and stocks of on-farm soil organic matter on the long-term welfare of these households. We find that larger farms are better able to cope with both labour shocks and deteriorating natural capital than smaller farms, with smaller farms remaining poor and unable to invest into more diversified agricultural activities, like livestock. This suggests locally increasing returns to various combinations of economic and biophysical assets. Information obtained through such simulation model experiments may lead to better targeting of poverty alleviation programs as well as suggest a broader array of strategies that play off of the complex interactions between economic and biophysical assets. The flexibility to examine these different leverage points is partially provided by our use of system dynamics, rather than other modeling techniques, to develop a descriptive rather than prescriptive model of farm behaviour.

Raising agricultural productivity in smallholder agriculture in sub-Saharan Africa requires an understanding of if and how farm household land use and socioeconomic factors affect soil fertility. Market access, population growth, socio economic characteristics and agro ecological zones have been proposed as important drivers of land use intensity and, consequently, soil fertility. We used diffuse reflectance infrared spectroscopy to measure soil fertility, and multivariate and exogenous switching regression statistical approaches to determine if soil fertility in the smallholder farms of the highlands of Kenya is associated by region, land use categories (cash crop, food crop, fodder and pastures), and selected household socio economic factors (household income, number of adults, farm size and number of cattle). Over 2000 fields on 236 farms were sampled in Embu (eastern Kenya highlands, primarily Andosols) and Madzuu (western Kenya highlands, primarily Ferrasols). Soil fertility variables, including total soil carbon (SC), total nitrogen (TN), pH, available Olsen phosphorous (P), extractable potassium(K), calcium (Ca), and magnesium (Mg), effective cation exchange capacity (ECEC) and texture, were measured using conventional laboratory techniques on 15% of the sampled soils. From these analyses, SC, TN, P and K were all greater in Embu compared to Madzuu soils. Soil fertility variables were significantly higher in pastures compared to other land uses in Madzuu, but were comparable with other land uses in Embu. This soil data was then used to calibrate soil reflectance results in order to predict soil fertility variables for all soil samples. Principle component analysis (PCA) of soil fertility variables developed from the spectroscopy data for each soil sample indicated similarities among sites in the three most important eigenvectors: the first (soil nutrient) vector had high positive loadings for K, Ca, Mg, ECEC and pH; the second (soil organic matter, SOM) vector had high positive loadings for soil organic carbon and total nitrogen; and the third (soil texture) vector had high positive loadings for clay plus silt. However, in Embu, P was associated with the soil organic matter vector while in Madzuu it was associated with the soil nutrient vector. In comparison to pasture all other land uses were associated with lower values of soil nutrient and SOM components in Madzuu, while in Embu, these other land uses were associated with higher values of the SOM component. Number of cattle per farm had no association with any of the three soil fertility components at either site. In Embu, farm income and adult population were both positively related to SOM. In Madzuu, farm size was positively associated with SOM but negatively associated with soil nutrients. More than twice as much P fertilizer is applied on average in Embu compared to Madzuu (27 vs. 11 kg ha-1 season-1). Our study supports the link between poverty dynamics and soil degradation in smallholder agriculture; wealthier households in the eastern Kenya highlands are able to invest in soil fertility management while the poorer households in western Kenya are mining nutrients in soils.

That farmers rely on the land for their livelihoods is obvious. The converse, that ecosystem services depend on farmers' behaviors, must also be recognized if agricultural productivity is to be improved. In sub Saharan Africa, the 70% of the population employed in the agricultural sector (Sanchez 2002) is engaged in an on-going 'dialogue' with the agricultural natural resource base. Recently, this conversation has not been going well: per capita food production has remained stagnant for the last 40 years so now 180 million on the continent lack adequate food, a number that has increased by 100% since 1970 (Sanchez 2002). To provide adequate diets to the African population, increases in crop yields of 3.0 to 3.5% y-1 are needed (Reardon et al. 2001), but such increases have not been realized as average maize yields have remained static at 1200 kg ha-1.