CHAPTER 5: PRE-INCUBATION OF DUCKWEED BIOMASS IMPROVES NITROGEN
5.5 Discussion
The increase in Swiss chard biomass with increasing rates of duckweed application could be explained by relative increase in nutrient uptake, especially N and K, compared to the control.
Nitrogen uptake of Swiss chard generally increased emphasizing the essential role of N in plant growth (Engelbrecht et al., 2010). The similarity of soil residual N at all rates for the individual soils suggested that the increase in supplied N, as a result of duckweed addition, made N more available for plant growth, with limited effects on residual levels.
87 Table 5.8 Effect of period of duckweed incorporation on residual soil chemical properties after Swiss chard grown in the ferralsol
Biomass Pre- Inc
C N P K Ca Mg EA pH Mn Zn Cu
(days) (%) (mgkg-1) (cmolkg-1) (mgkg-1)
Control 3.10 0.22 20.7ab 0.37 4.99 1.77c 0.43 4.23bc 34.2 5.81ab 5.29 +Control 3.31 0.27 20.9ab 0.33 4.99 1.57abc 0.51 4.16ab 34.5 5.70a 5.50 LM 0 3.22 0.25 20.2ab 0.39 5.18 1.81c 0.39 4.29c 36.5 6.39ab 6.03 LM 14 3.19 0.26 22.6b 0.36 4.94 1.62bc 0.37 4.22bc 30.2 5.55a 5.26 LM 28 3.21 0.25 18.4a 0.26 5.08 1.45ab 0.56 4.14ab 31.2 5.70a 5.61 WF 0 3.09 0.24 19.6ab 0.38 4.93 1.67bc 0.37 4.27c 36.6 6.69b 5.50 WF 14 3.06 0.23 21.5ab 0.33 5.00 1.62bc 0.46 4.19abc 32.8 5.66a 5.40 WF 28 3.20 0.25 19.1a 0.22 4.81 1.32a 0.53 4.10a 32.9 5.67a 5.00
Means followed by the same letter in a column are not significantly different at p<0.05. Means without superscripts are not significantly different at p<0.05. Pre-inc = pre-incubation period, LM = L. minor, WF= W. arrhiza, +Control= Urea applied at recommended rate at transplanting and 3 weeks after transplanting, EA= Exchangeable acidity
88 The highest dry matter of Swiss chard in the positive control at 100 kg N ha-1 and similar content in the duckweed treatments at 100 and 200 kg N ha-1, were in agreement with Hammad et al.
(2007) and Kołota and Czerniak (2010). Hammad et al. (2007) reported that nitrogen levels and sources influenced dry mass of spinach, while other studies generally maintained that the source of N did not influence the yield of leafy vegetables (Wang and Li, 2004; Engelbrecht et al., 2010).
In this study, application of urea provided readily available N for uptake (Brito et al., 2012), resulting in higher dry matter yield in the positive control, whereas duckweed biomass decomposed with gradual N mineralization. Kołota and Czerniak (2010) reported that high N rates between 150 and 200 kg N ha-1 did not improve dry matter yield of Swiss chard and increment of N dose from 100 to 200 kg N ha-1 did not cause any substantial yield enhancement (Kołota et al., 2017). The similarity of uptake of most nutrients in this study at the 100 and 200 kg N ha-1 implied that N uptake generally influenced the growth of the vegetable and affected the uptake of other plant essential nutrients in a similar manner such that there was no comparative dry matter advantage at 200 kg N ha-1. The increased available N from duckweed decomposition at higher rates and the positive control supported greater plant growth (Table 5.3), which facilitated greater uptake of other essential nutrients, than at lower rates where biomass accumulation was limited.
This effect was evident with uptake of K, Ca, Mg, Mn, Zn and Cu. The lower uptake of these elements in treatments, where dry matter was low, was supported by the higher levels of these elements in the residual soils, including phosphorus. The higher nutrient uptake by Swiss chard on ferralsol were consistent with its relatively higher fertility status than regosol (Tables 5.1 and 5.4).
Uptake trends of Fe by Swiss chard were not influenced by N rates for regosol due to soluble Fe at prevailing soil pH (<4.3), (Brady and Weil, 2008; Ranade-Malvi, 2011). This finding was supported by the relatively higher uptake of P in the regosol than ferralsol, where availability could
89 have been limited by fixation. The relatively lower soil pH, higher clay content and availability of soluble Fe and Al could have resulted in more P fixation (Lucas and Davis, 1961) by ferralsol than the regosol. Relative to the control, addition of higher rates of N from duckweed and urea contributed to soil acidification due to possible nitrification. Sanchez-Monedero et al. (2001) observed a decline in pH during organic waste composting due to nitrification and Turmel et al.
(2015) confirmed that the overall effect of N mineralization is acidifying. In preliminary incubation experiments, mineralization of N and its subsequent nitrification resulted in pH decline in the same soils used in this study. While the increasing duckweed rate up to an equivalent of at least 100 kg N ha-1 increased yield, the dry matter was still lower than the positive control, which suggested that some N was yet to mineralize to be available. This view was supported by the findings of the experiment where the duckweed was pre-incubated before Swiss chard was planted.
Pre-incubation of duckweed for 28 days might have been essential in making enough N uptake and all other nutrients, resulting in similar Swiss chard dry matter with the positive control. The period is appropriate for mineralization of N in duckweed as most nutrients are initially unavailable and have to be slowly released through microbial degradation (Brito et al., 2012). This observation was confirmed by lower dry matter of Swiss chard in the negative control and both duckweed species incorporated at transplanting (not pre-incubated) than the same species pre-incubated for 28 days. This finding indicates that in order to derive maximum benefits from duckweed as a nutrient source, pre-incubation is required. Pre-incubation for 14 days (two weeks) proved to be less effective at influencing Swiss chard dry matter probably due to initially low amounts of mineralized nutrients and lack of synchrony between N crop demand and N mineralization (Sainju et al., 2006). The ferralsol in this study was highly weathered, well drained and fertile (Table 5.2) and also provided NO3- through nitrification as indicated by residual soil pH that mostly declined
90 from duckweed incorporated at transplanting to that pre-incubated for 28 days. The higher dry matter of Swiss chard at longer pre-incubation periods, in response to greater mineral N, was accompanied by great uptake of other plant essential nutrients from both the soil and incorporated duckweed biomass including K, Ca, Mg, Mn and Zn, especially for W. arrhiza.
Generally, high uptake of nutrients by Swiss chard after pre-incubation of W. arrhiza for 28 days could be explained by the amendment’s elemental composition. W. arrhiza had a narrower C:N ratio and higher N content than L. minor and this could lead to the former decomposing more rapidly in soil and releasing nitrogen (Kumar and Goh, 2000; Tejada et al., 2008) that was readily taken up by the plants.
The non-response of P uptake of Swiss chard over period of pre-incubation indicated limited ability of the plant to utilize the nutrient as decomposition of duckweed proceeded. Conversely, as highlighted in Chapter 4, Section 4.4.5, high Fe and Al levels in the ferralsol at relatively low pH coupled with additions from duckweed decomposition and the acidifying nature of the nitrification process could have resulted in P fixation on Al and Fe oxyhydroxide surfaces and precipitation of Al and Fe phosphates (Lucas and Davis, 1961). This might have affected P uptake as confirmed by the inorganic fractionation of P in Chapter 4 (Section 4.4.5), and generally similar residual soil P values and even lower for the 28 day incubation period, where residual soil pH was lowest.
Although all treatments were corrected for K, the pre-incubated treatments had higher K uptake by Swiss chard from W. arrhiza pre-incubated for 28 days than when not pre-incubated, possibly as a result of synergistic effects with the higher N released by mineralisation (Ranade-Malvi, 2011). This trend is not exhibited by L. minor treatments possibly due to high Ca levels, released from its biomass, that antagonized uptake of K.
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