CHAPTER 4: DECOMPOSITION OF WOLFFIA ARRHIZA RESIDUES RAPIDLY
4.5 Discussion
The production of at least 5% ammonium-N on the first day of incubation implied a material that readily decomposes, and this could be explained by the low C/N ratio of around eight, below the threshold value of 20-30, signifying net mineralisation (Kumar and Goh, 2000).
Rapid decomposition may be facilitated by low fibre and lignin content as the macrophytes do not have upright structures that need mechanical support (Hasan and Chakrabarti, 2009; Tao et al., 2017). Generally, the peak ammonium-N production was at two weeks of incubation and deviations were due to activities influenced by rates of duckweed application and soil types.
The peak ammonium-N production for Baynesfield A soil after a week at higher duckweed rates might be due to stimulation of a diversity of heterotrophs supported by the relatively more fertile soil (Table 4.1). The rapid ammonification could be accompanied by volatilisation, with possible N losses to the atmosphere. The ammonium-N concentration of lower rates of duckweed, unlike high ones, for relatively fertile soils quickly declined. The higher ammonium-N at higher duckweed rates at different incubation periods, especially within the first 28 days, could be explained by conversion of higher levels of organic-N added as a component of large quantities of duckweed. Decline in ammonium-N production after the first two weeks of incubation, for all soils except the Baynesfield B, could be explained by the inception of the nitrification phase under moist and aerated conditions (Figure 4. 2). In relation to the trends of ammonium-N, it suggests that both ammonium production and nitrification occurred at the same time up to 28 days. This is supported by the magnitude of nitrate-N produced that was much higher in soil (up to 500 mgkg-1) than that of ammonium-N (up to 150 mgkg-1).
The general increase in nitrate-N and mineral-N from days 14 to 42 corresponded with a decrease in ammonium-N. This explains the nitrification process engendered by a conducive
67 environment of soils at 25oC and field capacity moisture content (Sahrawat, 2008). Whilst the Baynesfield B soil maintained a high ammonium-N concentration, its conversion to nitrate-N was sluggish, probably attributable to low initial populations of nitrifying organisms (Tisdale et al., 1993) in the subsoil that had a relatively high clay content. In addition, Brady and Weil (2008) highlight the importance of cations, phosphorus and micronutrients in stimulating nitrification and in contrast, the Baynesfield B soil had lowest amounts of these elements (Table 4.1). As such, infertile soils may result in lower nitrification of N from duckweed biomass added to soil. The higher nitrate-N at higher duckweed rates at different incubation periods could be explained by conversion of higher levels of ammonium-N from the large quantities of duckweed added.
The more rapid release of mineral-N from the Baynesfield A and Ukulinga soils, in 42 days, and the slower increase in Baynesfield B suggested that inherent soil fertility is an important factor. Soils of low fertility status would mineralise N from this amendment at a slower rate than those of higher fertility, possibly because of low initial microbial biomass. This would suggest that pre-incubation of duckweed will be necessary in order to synchronise crop nitrogen requirement and N availability (Malepfane and Muchaonyerwa, 2017) in soils of low fertility.
Mineralisation of duckweed could have been facilitated by the availability of other nutrients such as P. The C:P ratio of about 68:1 implies net mineralisation of P though extractable-P declined with duration of incubation. The net P release was consistent with the C:P ratio rule of thumb where mineralisation (C:P ≤ 200) prevailed during decomposition (Sharpley and Smith, 1989; Dossa et al., 2009). Depending on rate of duckweed application, at least 16% to about 40% of duckweed P extractable from the soil on the first day of incubation suggested high water soluble P in the plant material that immediately released high levels of P after
68 incorporation in soil as supported by Kwabiah et al. (2003). In a study on addition of organic and inorganic P sources to soil, Malik et al. (2012) reported that high P residue addition increased the concentration of fractions such as labile P within 5 hours of addition to soil, which they attributed to soluble P within the residue. Landolt and Kandeler (1987) highlight that duckweed is exceptional among macrophytes as they store P as orthophosphates within the vacuole, as condensed inorganic phosphates and in phytic acid. The rupturing of the duckweed cells on mixing with soil at field capacity moisture level could have contributed to high water soluble P at the start of the incubation. The decline of extractable-P over the incubation period for all soils was similar to trends observed by Malik et al. (2012), where their concentration of the labile resin and hydrogen carbonate inorganic P for most treatments with high P content in amendments decreased from days 0 to 56. The authors suggested transformation of inorganic P from labile into non-labile through sorption/fixation. In this study, higher concentrations of the NaOH extractable-P pool than all other pools from the sequential extraction suggested Fe and Al from both the duckweed and the acidic soils transformed most of the mineralised P from W. arrhiza into the non-labile form. The duckweed had high tissue Al and Fe content whose combined percentage (0.5%), was slightly less than its tissue P content. The results show that the Fe and Al content of duckweed species could have a significant influence on lability of mineralised P. This suggests need for lime addition before decomposition of high Al and Fe bearing duckweed.
Although extractable-P (available P) declined with incubation time, it was higher in the Ukulinga soil, possibly due to lower clay content and higher pH (Table 4.1), and was supported by the higher ammonium chloride-extractable pool during the fractionation (Table 4.2). The Ukulinga soil had relatively higher ammonium chloride P pool than the Baynesfield soils after 56 days due to lower clay content. The rates of duckweed application did not influence the
69 H2SO4 extractable-P since the 1% rate had almost no extractable-P, indicating little significance of Ca bound P.