Assessment of soil quality across differently aged, naturally recovered old crop fields and natural sites in a Grassland biome

doi:10.21203/rs.3.rs-1753614/v1

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Assessment of soil quality across differently aged, naturally recovered old crop fields and natural sites in a Grassland biome

https://doi.org/10.21203/rs.3.rs-1753614/v1

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Cropland abandonment for natural recovery has increase globally over the las few decades. Changing land-use practices in South Africa, has led to an increased rate of cropland abandonment. The rate and magnitude of soil quality recovery following abandonment varies from location to location. Several soil quality studies have been conducted in the Eastern Cape Province of South Africa, mainly focusing on the impact of different cropping strategies on soil quality or the effect of different grazing practices on soil quality. Only a few studies on the effect of cropland abandonment and soil quality have been performed. We, therefore, aimed to assess the change in soil quality of differently aged, recovering old crop fields compared to the surrounding natural veld. The study site is located in the Winterberg Mountains of the Eastern Cape, South Africa. Soil characteristics, including soil water-holding capacity, density, pH, total dissolved solids and electrical conductivity, C %, N %, C stock, N stock, as well as sand, silt clay composition was assessed for three recovering old crop fields. Samples from the surrounding natural habitat were also included for comparison. Significant positive change in soil water-holding capacity (WHC), C %, N %, C stock and N stock were observed with increased age since abandonment. It is clear from our results that soil recovery is taking place, with the older former crop field most closely resembling the natural habitat. It will, however, still take a significant amount of time for total recovery to be achieved. Continuous monitoring of old crop fields in agricultural, as well as, formally protected areas is needed to fully understand the long-term effects of cropping on soil quality in this region.

Significance

• Cropland abandonment is a global occurrence, with old croplands allowed to recover to a natural state.

• The effect of old cropland abandonment on soil quality in the Eastern Cape Province of South Africa is not well studied.

• No significant differences in pH and electrical conductivity was observed between differently aged old crop fields, but an increase in water-holding capacity, % soil N and % soil C was observed with increase old crop field abandonment age.

• These results contribute to the limiting amount of data on soil physical characteristics natural recovery of old crop fields in the Eastern Cape of South Africa

Agroecology

soil health recovery

old croplands

water-holding capacity

total nitrogen

total carbon

Over the last few decades, cropland abandonment for natural recovery has increased worldwide due to changing weather patterns and shifts in the rainy seasons (1–4). Climate change has also greatly affected crop farming in South Africa, where 13.7% of land is used for intensive agriculture (5). Many croplands are, however, being abandoned due to changing land-use practices and utilised for livestock grazing (6), or through the establishment of nature conservation areas (7, 8). The rehabilitation process of these old crop fields is generally approached through either active (i.e., seed planting of natural plants; 9) or passive rehabilitation (i.e., natural recovery; 10). The assessment of soil quality, in terms of nutrients and texture, are good indicators of cropland rehabilitation as it is well known to be negatively affected by crop farming practices over time (11, 12). The loss of topsoil and nutrients through wind erosion (13), as well as rain runoff are known causes of soil quality deterioration (14) which can be linked to commercial agricultural activities. Soil nutrient quality degradation can further be associated with the loss of the natural vegetation, and related biodiversity (14).

The positive effects of both active and passive vegetation regeneration on soil quality parameters are well documented. Huang, Zeng (15) reported significant increases of soil total nitrogen (TN) and total phosphorus (TP) following vegetation recovery during a meta-analysis of 101 published field sites. Soil organic carbon (SOC) and mineralized soil total inorganic N (MTIN) were shown to increase with the years of fallow in old fields of between 1 to 40 years in the Bolivian Andean Highland Region (16). In South Africa, Ruwanza (8) showed that micro-topography within Renosterveld old fields play a crucial role in native Renosterveld species recovery, with higher levels of soil fertility observed in furrows compared to ridges. These results further supported similar observation by Memiaghe (17) at abandoned fields in the Western Cape Provinces. Many other studies have also shown how soil quality can improve following cropland abandonment across the world, from the Canadian prairies (18), the Canary islands (19), to Australia (20). The rate of recovery is, however, governed by numerous environmental and anthropological factors such as annual rainfall, which can lead to soil erosion (21), as well as previous agricultural practices such as fertiliser use (22).

The Eastern Cape Province of South Africa is a highly mountainous region. Extensive mountain ranges, forming part of the Great Escarpment (23), as well as the eastern limits of the Cape Fold Mountains (24) dominate the landscape. The Eastern Cape mountains are biologically highly diverse areas, with high levels of endemism observed (25, 26). Several soil quality studies have been performed in the province. Many South African studies focused on the influence of cropping, including the use of different cropping techniques, on soil quality (27–30). Some studies also assessed the change in soil quality due to factors such as soil erosion (31, 32), woody vegetation encroachment (33) and change in vegetation type (34), which can be linked to improper livestock management practices.. Shackleton, Shackleton (35) performed a soil health assessment of a series of old crop fields in the Wild coast region of the Eastern Cape (South Africa). The Wild coast is a coastal, high rainfall area. These authors found a significant increase in carbon levels as the age of the old crop fields increased. A significant change in soil compaction was also observed, with older fields showing lower levels of compactness (35). This is one of the few studies on the change in soil quality following crop field natural passive recovery available from the Eastern Cape Province of South Africa.

The aim of this study was, therefore, to assess the possible variation in soil quality between differently aged, recovering old crop fields and the surrounding natural veld, in the Winterberg Mountains of the Eastern Cape, South Africa. This was accomplished through the measurement of pH, total dissolved solids (TDS), total carbon (C), total nitrogen (N), water-holding capacity, soil density and carbon and nitrogen stock across the differently aged, naturally recovering fields, as well as several natural veld sites. In contrast to the Shackleton, Shackleton (35) study, our site is located in a mountainous region with a lower annual rainfall record. This research will add to the growing amount of data on old field recovery in South Africa and will also provide valuable data for future biodiversity projects planned for this mountainous region.

Sample site

The sample sites are located on a private farm (Leliekloof farm) situated within the Great Winterberg mountain range, 55 km south-east of Cradock, Chris Hani District, Eastern Cape (32°19'37.4"S, 26°01'08.0"E). The naturally recovering old crop fields selected for this study are located on a plateau region on the farm at an average elevation of 1,658 m above sea level (m.a.s.l.). These crop fields were first established in 1963 (J.M. Coetzer, personal communication, April 2021). This area falls in the Grassland Biome and is represented by the Karoo Escarpment Grassland vegetation type (mixture of shrubs and grasses; 36). The geology of the region is represented by the Adelaide subgroup of the Beaufort Group of sedimentary layers, as well as Karoo Dolerite intrusions (37). The average annual rainfall for the region is 547 mm per year (1970 - 2000; 38), with the average rainfall over the last 5 years for the farm recorded as 365 mm (J.M. Coetzer, personal communication, April 2021).

On the study site several naturally recovering old crop fields reside which have not been planted or worked since 1989, 1997 and 2009 respectively (Fig. 1). These old fields have started to slowly recover in terms of natural vegetation. The old fields are surrounded by natural grassland, which has never been used for croplands, and the whole camp is used for grazing by sheep, goats and cattle, on a rotational grazing system. Upon visual inspection it can be seen that the old crop field sections show a difference in vegetation composition compared to the neighbouring natural areas. The old crop fields are mostly dominated by Eragrostis grasses and Chrysocoma ciliata dwarf shrubs, whereas the surrounding natural veld is dominated by Merxmuellera (Tenaxia) disticha and Themeda triandra grasses.

Soil sampling

Four sampling localities were selected for this assessment. Soil samples were collected from three sites in each of three recovering old crop field sections of three age groups (1989, 1997 and 2009) and a control site of natural grassland (Nat) bordering the fields, which has never been used for croplands. Each soil sample consisted of three subsamples to account for any variability within each locality, and collected in a 20 m transect, at 10 m intervals. Each subsample was collected with an auger to a depth of 10 cm. This gave us a total of 12 soil samples across all four localities.

An additional 12 samples were collected with a 4.5 cm diameter PVC pipe corer, to a depth of 4 cm. The samples were immediately placed in separate sealable bags, to minimize water loss. These samples were used to determine soil density. Each sample was taken by carefully pushing the corer into the soil and limiting soil disruption as much as possible.

Soil analysis

The 12 composite samples were dried at 60 °C for 72 h prior to analyses. All samples were sieved (2mm) to remove stones and large plant material prior to drying. Soil texture was assessed by determining the sand, silt, and clay composition of each composite sample following the Hydrometer Method (39). Soil pH (1:2 soil to water suspension) and TDS measurements were performed by weighing out 15 g of soil into a 50 ml falcon tube and adding 30 ml of reverse osmosis (RO) water. The mixture was thoroughly mixed, and the pH, TDS and electrical conductivity (EC) measurements were taken using a ExStik ® II pH/Conductivity meter (Extech Instruments, Waltham, MA, USA).

Soil density were determined by calculating the volume of each sample using the known corer diameter and depth sampled, as well as the dry weight of each sample. These samples were dried at 100 °C for 72 h to ensure total evaporation of any soil moisture. The soil bulk density (BD = g/cm³) was calculated by dividing the dry weight (g) by the sample volume (cm³; V = πr²h).

Total C and total N was measured from the sieved composite samples by dry combustion (40) using a TruSpec Leco CN analyzer (LECO Corp., St Joseph, MI, USA). The results were obtained as carbon percentage (C %) and nitrogen percentage (N %). Carbon and nitrogen stock values were calculated using the C % and N % estimates and the soil density values. The total amount of carbon and nitrogen per hectare (tC/ha and tN/ha) were calculated by multiplying the sampling depth, the calculated soil density and the C % and N %, respectively.

The soil water-holding capacity (WHC) was measured by calculating the amount of water present in a completely saturated soil sample. First, a Whatman® qualitative filter paper (Grade 113; 30 µm; ø 125 mm) was saturated in reverse osmosis (RO) water, weighed, and then folded and placed in a clean funnel. The funnel was then placed in a collection container to catch the water filtrate. Twenty grams of dried soil was added to the filter, with 25 ml RO water carefully added to the soil. The funnel was covered with foil to reduce evaporation, with five small holes punched into the foil to facilitate drainage. The soil was then allowed to drain for 30 min at room temperature. The saturated soil sample was then weighed with its filter. The weight of the filter and the initial soil weight was subtracted to calculate the amount of water. The value for the amount of water was divided by the initial soil weight and multiplied by 100 to calculate the water holding capacity.

Statistical analyses

A Mantel test (41) was performed to test for spatial autocorrelation in our data, using the ade4 package (42) in R (43). The GPS coordinates of each sample site was used to calculate a distance matrix for comparison to a distance matrix estimated from all soil characteristics. The soil characteristics measured from this study were compared between old crop field age groups (1989, 1997 and 2009) and the natural grassland using a Multivariate linear regression assessment in PAST v4.01 (44). The measurements for soil density, pH, TDS, N %, C %, N stock, C stock, WHC and soil texture (sand, silt, clay) were set as dependent variables. The age class was set as the independent variable. The ages since abandonment was calculated for each site, with an age of 60 years selected for the natural habitats, as the first crops were planted in 1963 (J.M. Coetzer, personal communication, April 2021). Setting the age of the natural habitats to 40, 100 or 150 did not have any significant changes to the regression analysis results.

Non-metric multidimensional scaling (NMDS) was performed to visualise similarities between treatments using the R package VEGAN (45, 46). The NMDS plot was produced using a Gower’s dissimilarity matrix (47) produced from all calculated measures. Gower’s index was selected as it has been shown to be reliable when using several types of variables (48, 49).

The Mantel test showed no significant correlation between soil characteristics and spatial distance (p = 0.114). The summary statistics of all parameters are provided in Table 1. The multivariate linear regression analysis indicated that the soil WHC, N %, C %, N stock and C stock values were all positively correlated with age since abandonment (p < 0.005; Table 2; Fig. 2). The estimated mean values of all five these measures showed an increase when moving from the youngest abandoned field sites, to the oldest and then the natural sites (Fig. 3).

The observed differentiation between the naturally recovered old crop fields and natural sites was further supported by the NMDS analysis (Fig. 4). Goodness of fit (Gof) values indicate a good fit of the observations on the NMDS plot (Gof values < 0.01), with a stress value of < 0.05 (non-metric fit r² = 0.999; linear fit r² = 0.996). No overlap was observed between the recovering old crop fields and the natural veld. The old crop fields did, however, show overlap as well as signs of variation within the age groups. It can be seen form the NMDS x-axis that there is a progression from the youngest of the recovering crop fields to the natural sites, with the oldest crop field grouping closest to the natural sites. This provided further support for observations from the post-hoc tests performed during the MANOVA analysis.

Our current study aimed to establish the change in soil quality of former crop fields that have been left to naturally recover situated in a mountainous grassland ecosystem over time. No significant differences were observed between the natural sites and the three old crop field groups when comparing soil density, pH, salinity (TDS) and soil texture parameters. These observations cold indicate that the old crop fields have already started to revert to the natural conditions, although not completely. Soil WHC, C %, N %, C stock and N stock did differ between the natural sites and the old crop fields.

The average pH levels, as well as the soil texture measurements recorded in our study were similar to that of the non-degraded soils observed by Oluwole and Sikhalazo (31) at Tsolwana Nature Reserve (NR) in the Eastern Cape. The soil texture results from our current study identify the soil as a sandy loam texture (50). Tsolwana NR is located ~ 45 km north-east of our sample site and is geologically located in the Tarkastad subgroup (Beaufort group; 37). A decrease of soil pH from extremely degraded to non-degraded soils were observed at Tsolwana NR (31), whereas we observed no significant differentiation between our samples site. Much higher pH levels were observed for crop fields at the University of Fort Hare Research Farm near Alice (Eastern Cape), which employ different rotational cropping systems (27). Aguilera, Motavalli (16), similarly to our study, observed no significant difference between pH or electrical conductivity while investigating changes in soil quality of 10 to 40-year-old abandoned crop fields in the Bolivian Andean Highland Region. This could be an indication of pH levels remaining stable over time at our sampling sites. No signs of erosion were observed in the fallow fields located in our study site. The retention of the field contours and the natural establishment of a mixture of grasses and forbs could have prevented erosion and excessive loss of topsoil following abandonment of crop farming. Additional to the evident positive effects gained from the retention of old field contours in terms of soil erosion prevention, it was also previously reported that increased soil fertility is observed within the furrows left in old fields (8, 17). These furrows could then play a crucial role in native vegetation colonisation from the surrounding natural habitat. The colonisation by native vegetation types could therefore also have assisted in the recovery of pH and salinity over time in our study sites.

The trend observed for the WHC estimates in our study can be linked to denser vegetation levels observed in the natural sites and the gradual natural restoration of vegetation in the fallow fields. The denser vegetation levels will in turn provide a larger organic matter component to the soil, decreasing soil compactness and in turn influencing WHC (50). This is further supported by the higher C %, N %, C %C stock and N stock values observed for the natural sites, and the increase of these values with field abandonment age. A significant increase in soil C and a decrease in soil compactness as the age of old crop fields increased was also reported by Shackleton et al. (2013) while assessing old crop fields in the low altitude and high rainfall region of the Wild Coast, Eastern Cape. Vegetation restoration has been linked to increases in C (51, 52) and total N (15), with higher vegetation diversity levels associated with higher soil C and N levels. The observed difference in C and N levels among the sample sites can also be linked to the difference in vegetation diversity observed between the sites (51). It has been reported that different plant groups (e.g. C3 vs C4 grasses) deposit C and N into the soil at different rates due to higher root biomass and slower root degradation observed in C4 grasses compared to forbs and C3 grasses (53). The observed increased C and N estimates can further be related to the presence of sheep which graze these camps on a rotational basis. Livestock is known to contribute positively to C and N levels in soil through urine and faecal secretion (54–56). In the long term, total recovery of our study sites to a natural state will likely not occur for a significant amount of time. Therefore, continued monitoring of this site and other similar sites located in agricultural and conservation areas in the region is imperative to ensure proper habitat recovery.

It is clear from our results that the WHC, C and N levels of these recovering old crop fields will only fully recover in another few decades. It is therefore important to continuously observe such sites to prevent the establishment of invasive species and promote the colonisation of native vegetation. This is especially true for wildlife conservation areas located on old agricultural lands. Further research focusing on how soil heath recovery affects vegetation and animal biodiversity are important aspects to also consider when investigating habitat recovery. Such research will further contribute to the available information on old crop field habitat recovery in South Africa, providing valuable resources to conservation biologist both locally and globally.

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Table 1. Summary statistics for all soil parameters measured during this study. The GPS coordinates of each sample site is also provided. (WHC – water-holding capacity; TDS – total dissolved solids; EC – electrical conductivity; %N - percentage soil nitrogen; C % – percentage soil carbon)

Sample ID	Type	Soil WHC	Soil density	pH	TDS (mg/L)	EC (µS/cm)	N %	C %	N stock (tN/ha)	C stock (tC/ha)	% Sand	% Silt	C %lay	Latitude	Longitude
Nat_1	Nat	52.815	1.543	6.080	250.000	357.143	0.219	2.615	3.381	40.342	87.000	11.000	2.000	-32.329239	26.017167
Nat_2	Nat	58.917	1.892	5.810	226.000	322.857	0.232	3.229	4.380	61.084	83.000	15.000	2.000	-32.328033	26.017499
Nat_3	Nat	56.209	1.811	5.870	219.000	312.857	0.200	2.571	3.614	46.565	91.000	7.000	2.000	-32.327047	26.018887
1989_1	1989	49.927	1.704	6.000	217.000	312.857	0.172	2.190	2.939	37.326	81.000	15.000	4.000	-32.335250	26.015162
1989_2	1989	52.733	1.694	5.820	217.000	328.571	0.193	2.408	3.268	40.783	88.000	11.000	1.000	-32.335131	26.016908
1989_3	1989	50.730	1.534	5.950	220.000	302.857	0.177	2.101	2.711	32.233	89.000	8.000	3.000	-32.333117	26.017632
1997_1	1997	50.817	1.600	5.980	224.000	320.000	0.144	1.792	2.304	28.665	92.000	4.000	4.000	-32.331008	26.017114
1997_2	1997	50.000	2.312	5.730	221.000	315.714	0.124	1.609	2.874	37.204	76.000	17.000	7.000	-32.330397	26.015632
1997_3	1997	45.554	1.527	5.470	235.000	335.714	0.163	1.878	2.487	28.677	81.000	16.000	3.000	-32.329344	26.016800
2009_1	2009	41.700	1.611	5.800	219.000	310.000	0.098	1.164	1.573	18.751	90.000	8.000	2.000	-32.335483	26.014449
2009_2	2009	43.560	1.773	5.840	230.000	310.000	0.174	2.108	3.081	37.361	87.000	11.000	2.000	-32.335614	26.017917
2009_3	2009	42.348	1.916	6.020	212.000	314.286	0.133	1.708	2.556	32.722	92.000	6.000	2.000	-32.333608	26.018612

Mean		49.609	1.742	5.864	224.167	320.238	0.168	2.115	2.930	36.809	86.417	10.750	2.833	-	-
SE		1.561	0.065	0.047	2.956	4.223	0.012	0.158	0.205	3.027	1.469	1.232	0.458	-	-

Table 2. The multivariate linear regression results assessing the effect of time since abandonment between three old crop field age groups and natural grassland. The Mantel test r values and significance values (p-value = 0.05) are provided.

Independent variable	Dependent variables	r	p-value
Age	soil density	-0.059	0.855
	pH	0.200	0.533
	TDS	0.383	0.220
	N %	0.835	0.001
	C %	0.847	0.001
	N stock	0.785	0.003
	C stock	0.756	0.004
	WHC	0.895	0.0001
	sand	-0.065	0.841
	silt	0.150	0.641
	clay	-0.195	0.543

The authors would like to thank Dr Elmarie Kotze from the Department of Soil- and Crop- and Climate Sciences, University of the Free State, for her advice and guidance during this study. The authors would also like to thank Mr JM Coetzer for providing access to his property and allowing us to conduct our research there. The authors would also like to acknowledge the University of the Free State and the University of Fort Hare for providing the necessary resources and time to do this research.

Conflicts of Interest

The authors confirm that there is no conflict of interest related to this publication.

Data Availability Statement

All data used for our analyses are provided in Table 1.

Declaration of Funding

The study was funded by research incentive funding provided to WGC by the Department of Genetics, University of the Free State.

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Assessment of soil quality across differently aged, naturally recovered old crop fields and natural sites in a Grassland biome

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