Long-term fertilizer and sugarcane residue management effects on structural stability of two soil types in South Africa.
Mthimkhulu, Sandile Siphesihle.
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Under sugarcane production, soil aggregate stability (AS) is affected by the harvesting method i.e., burning, mulching and fertilizer application. This study combined mineralogical, biological, chemical and physical approaches to investigate the effect of these management techniques on a range of soil properties that may influence soil aggregation. The study site was located at the South African Sugarcane Research Institute (SASRI) at Mount Edgecombe near Durban, KwaZulu-Natal, South Africa. It is the oldest long-term, continuously monitored sugarcane production and soil management trial in the world, having been established in 1939. The area is characterized by summer (October to March) rainfall. Within the study site itself the dominant parent material was dolerite, with colluvial material in the south-western part of the lower slope. Due to the variation in topography, two soil types were identified. On the upper slope, the soil was classified as a Mollic Cambisol, locally known as Mayo form (Glenecho family). On the lower slope, the soil was a Mollic Nitisol, locally known as Bonheim form (Rockvale family). The trial is a split-plot factorial design arranged in a randomised complete block with four replicates for plots burned at harvest and eight replicates for all unburned plots. The main plot treatments are a) green sugarcane harvesting with all residues retained and spread evenly over the plot area (M), b) sugarcane burned prior to harvest (no foliage residue) with sugarcane-tops left scattered evenly over the plot area (BS) and c) sugarcane burned prior to harvest with all residue (sugarcane-tops) removed from the plots (BR). Split-plot treatments consisted of unfertilized (F0) and fertilized plots (F) receiving an annual application of 140 kg N, 28 kg P and 140 kg K ha-¹ as 5:1:5 (46). From the 32 plots, 24 were selected including four replicates of each of the treatments. Three replicate soil samples were collected with a spade at two soil depths (0-10 and 10-20 cm) from mini-pits in each of the 24 chosen plots. For soil AS determinations, samples were air-dried and sieved to collect soil aggregates between 2.8 and 5 mm and the mean weight diameter (MWD) determined. Some of the air-dried bulk sample was analysed for total carbon (Ct) and nitrogen (N), organic carbon (OC), pH, exchangeable calcium (Ca), magnesium (Mg), sodium (Na) and potassium (K), aluminium (Al), soil texture (clay, silt and sand content), clay mineralogy, soil microbiological properties, phosphorus (P), zinc (Zn), copper (Cu), and manganese (Mn). Soil microbiological properties (the abundance and communities of bacteria and fungi) were measured on the 0-10 cm depth samples only. The saturated hydraulic conductivity (Ks), bulk density (ρb), water retention and available water capacity (AWC) were determined on undisturbed soil cores also collected from 0-10 and 10-20 cm soil depths. Penetrometer resistance (PR) and apparent electrical resistance were measured in-field across the whole trial while the gravimetric soil moisture was measured in the laboratory and then mapped for the whole trial. Mulching and burning as well as fertilizer application showed no clear relationship with the clay mineralogy of the investigated soils. The main clay minerals in both soils were high defect kaolinite, vermiculite and lepidocrocite. The main difference in mineralogy found was that the upper slope soil also contained talc, illite and interstratified vermiculite-smectite which were not present in the lower slope soil. However, differences in clay mineralogy between the two slope positions had no influence on the other measured soil properties. The OC and Ct increased non-significantly (p > 0.05) in M and BS compared to BR in both fertilized and unfertilized treatments suggesting that the soils might have reached their equilibrium in terms of carbon. A significant increase (p < 0.05) caused by M treatment was, however, observed in N. The Ct and N were generally significantly higher (p < 0.05) in the macroaggregates compared to the microaggregates (0.1 - 0.05 mm) in most treatments, showing the direct contribution of soil organic matter (SOM) to the stability of larger aggregates. The Ca, Mg, pH and effective cation exchange capacity (ECEC) of the soils were similar between burned (BR and BS) and mulched (M) treatments but they decreased significantly (p < 0.05) in the fertilized treatments. Sodium concentrations were higher in the BRF0 and BSF0 treatments compared to the rest of the treatments. Potassium was significantly higher (p < 0.05) in MF0, and MF treatments compared to BSF0 and BRF0 treatments. The decrease in soil pH was mirrored by an increase in Al concentration and acid saturation in the fertilized treatments. These results could be due to the combined effects of basic cation mining by sugarcane plants, leaching of basic cations and their replacement by Al, mineralization of mulch leading to soil acidification, and oxidization of ammonium to nitrate. The higher concentration of P in the M treatments suggested that P resulted from both the fertilizer application and mineralization of SOM. High K accumulation came from the annual NPK fertilizer application. The dsDNA significantly increased (p < 0.05) in M compared to BR in the F0 treatments and remained similar between M, BS and BR treatments in the F treatments. It decreased significantly (p < 0.05) in the F compared to the F0 treatments. Although fertilizer application had no effect, M treatment significantly (p < 0.05) increased the abundance of bacteria and decreased the abundance of fungi 16S rDNA copy numbers. Bacterial richness significantly (p< 0.01) increased and decreased under mulching and fertilization, respectively, while the evenness decreased significantly (p < 0.01) in M and fertilized plots. Fungal richness significantly (p < 0.01) increased under M treatment in F0 treatments but showed no clear trend in the F treatments. Fertilizer application significantly (p < 0.01) reduced fungal richness. Burning and mulching showed no significant (p > 0.05) effect on fungal evenness though it was significantly (p < 0.01) decreased by fertilizer application. The MWD increased slightly in the following order: BR < BS < M under F treatments at the 0-10 cm depth, but the differences were not significant (p > 0.05). These results were associated with the lack of differences or consistent increase in soil aggregating agents observed between M and burned (BR and BS) treatments. The MWD was significantly (p < 0.05) reduced by fertilizer application possibly due to the decrease in divalent exchangeable bases (Ca and Mg) and fungal richness observed in this treatment compared to the F0 treatments. In the absence of a correlation between OC and MWD, the multivariate analyses showed that fungi were the main factor influencing AS though some significant effects of exchangeable bases were also found. The changes in MWD possibly induced by fertilizer application showed no effect on PR and the decrease in PR observed in the M treatments was attributed to an increase in moisture (due to higher SOM) compared to the burned treatments. Similarly to PR, bulk density, water retention and AWC showed no clear relationship with MWD. Therefore, the higher water retention found in BS and M treatments was attributed to the direct effect of SOM. The saturated hydraulic conductivity (Ks) decreased significantly (p < 0.05) in the fertilized treatments following the decrease in MWD. In conclusion, the long-term effect of mulching and burning on soil properties can be influenced by other external factors. In this study, the annual application of NPK fertilizer counteracted the impact of burning and mulching on AS and associated properties. Some of the properties were mostly influenced by soil type rather than sugarcane management practices. The annual application of NPK fertilizer also appeared to have led to increased acidification and soil structural deterioration (lower AS) under long-term sugarcane production regardless of the harvesting method practiced. Increasing additions of sugarcane residues are thus not necessarily sufficient to lead to improved soil structural stability and related soil properties.