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Kappaphycus alvarezii seaweed extract (KSWE) is known to enhance crop productivity and impart stress tolerance and our preliminary studies showed their biostimulatory effect on soil bacteria also. Close to one quarter of the foliar spray carried out on maize falls on soil either as drift or from leaf as drip. Hence it was hypothesized, it would profoundly influence soil microbes under stress. An experiment was conducted with five treatments, with or without KSWE application at critical stages of maize under soil moisture stress and compared with an irrigated control. Illumina platform was employed for analysis of V3-V4 region of 16S rRNA gene from the soil metagenome. Total of 345,552 operational taxonomic units were generated which were classified into 55 phyla, 152 classes, 240 orders, 305 families and 593 genera. Shannon’s index and Shannon’s equitability indicated increased soil bacterial diversity by multiple KSWE applications under duress. The abundance of Steroidobacter, Balneimonas, Rubrobacter, Bacillus, Alicyclobacillus, Anaerolinea and Nitrospira genera decreased (49-79%) in drought imposed at V5,10, and 15 stages of maize over the irrigated control, while it significantly improved when followed by KSWE application under drought. Nitrosomonas, Nitrosovibrio, Rubrobacter, Flavobacterium genera and several other taxa which are important for plant growth promotion and nutrient cycling were found to be enriched by KSWE application under drought. Treatments having enriched microbial abundance due to KSWE application under stress recorded higher soil enzymatic activities and cob yield, suggesting the contribution of altered soil ecology mediated by KSWE as one of the reasons for yield improvement.

Fluoride levels in seven commercial phosphate fertilizers (four single superphosphate samples, two diammonium phosphate samples, and one ammonium nitrophosphate sample) were analyzed independently at three laboratories employing the techniques of ion chromatography and ion-selective electrode. The results were consistent for aqueous solutions containing 100 mg L−1 (ppm) of fertilizer. The average values of fluoride from four different studies varied from 0.140 ± 0.006–1.33 ± 0.158% (w/w) for the seven fertilizer samples. The [P2O5]/[F] ratios (w/w) were computed for all the seven samples and the values were in the range of 13.79–328.57. By comparing these values with the average [P2O5]/[F] ratio in phosphate rock, it was inferred that 3–75% of fluoride originally present in rock remained in the fertilizers. IR spectral data revealed a small peak at 716 cm−1 – indicative of SiF62− – in the sample containing lowest fluoride which may be on account of fluoride stripping of intermediate phosphoric acid with reactive silica in this case. Considering 15 MMT (million metric tons) of annual consumption of phosphate fertilizers in India, the incremental load of fluoride in agricultural fields was estimated to be 127,650 ± 14,550 MTy−1 based on the grand average fluoride content of 0.851 ± 0.097% (w/w). While a part of this fluoride would likely get discharged into oceans through run-off, the remainder may persist in the soil, some amount may accumulate in vegetation, and a part might find its way into fresh water bodies, thereby aggravating the fluoride problem in the Country. The problem of non-point source pollution can be reduced by eliminating fluoride at source, and utilizing the recovered fluoride as feedstock. This could, in principle, satisfy the entire requirement of the fluorochemicals industry in India. Environmental costs need to be factored in while making an assessment of the viability of fluoride recovery and reuse in this manner compared to production from virgin sources of fluoride.

Kappaphycus seaweed extract (KSWE) has been reported to enhance the yield of several crops under normal conditions. The detection of the quaternary ammonium compounds (QACs) glycine betaine and choline chloride (GBCh) in KSWE impelled us to explore its efficacy on growth and yield under soil moisture deficit conditions. Another objective was to elucidate the extent to which the QACs have a role in mitigating drought stress, if at all. Thus, a factorial experiment was carried out on maize plants, wherein five foliar treatments (KSWE at 10 and 15%; GBCh at equivalent concentration to that found in 10 and 15% KSWE, respectively, designated as 10 and 15% Eq-GBCh; and water spray control) were subjected to three soil moisture stress levels (well-watered, WW; moderately stressed, MS; and severely stressed, SS). Observations were recorded for growth and yield parameters, pigments, photosynthetic attributes, antioxidant enzymes, and quality of grains. The results revealed the ability of KSWE to alleviate soil moisture stress, 10% KSWE being effective in increasing the seed yield under WW and MS conditions while 15% being optimal under SS condition. The percent increases in seed yield over their respective controls under WW, MS, and SS conditions at the optimal KSWE concentrations were 13.5, 21.7, and 36.4%, respectively, indicating higher grain yield response to KSWE treatments at higher stress levels. The yield advantage under stress could be attributed to minimal damage of photosystem in KSWE-treated plants as evidenced by higher pigment content, photosynthetic rate, reduced photoinhibition, and lipid peroxidation by enhanced protection against reactive oxygen species. The protein content in grains was enhanced by KSWE application under all stress groups compared to their respective controls. Although the predominant role of GBCh in KSWE towards drought mitigation and yield response was apparent, the results also connoted towards the role of other constituents in KSWE acting in unison along with GBCh, which should be investigated further.

Burgeoning population demands increased agricultural production; however, the climate change impact being currently experienced necessitates sustainable crop intensification. In this context, Kappaphycus alvarezii seaweed extract has potential to increase the crop productivity in an environmentally friendly way. Since K. alvarezii or rather any seaweed extract contains several beneficial constituents, we hypothesized that not all the stages of crop would respond identically upon its application. A literature survey indicated that there was no concrete rationale behind the frequency and application time of seaweed extract as it arbitrarily varied across the studies. Therefore, a field experiment was designed to evaluate the effect of seaweed extract at select crop growth stages of maize. The application time included ten different combinations chosen among V3, V5, V10, V15, and grain filling stages. The results corroborated our hypothesis, and it was demonstrated that application of a Kappaphycus seaweed-based bio-stimulant at the V10 stage was not effective in significantly influencing productivity. However, in contrast, when applied earlier at V5 or later at the V15 stage, a yield improvement of 13.7 and 11.1% was recorded over their respective controls. We also showed that application of the extract at V5 along with V15 elicited the best yield response (32%) and no further significant gains accrued with higher number of applications. In addition, there was a decline in the benefit-to-cost ratio from 1.7 recorded at V5 + V15 to less than or equal to 1 in other treatments. Consecutive application at V5 and V15 stages also showed a net 20% diminution in the climate change impact category compared to control, and this treatment resulted in savings of 35.7 kg CO2-equivalents per tonne of maize grain production compared to that where one application was done at the V10 stage only. Thus, the experimental result highlights the pre-eminence of the crop application stage at which seaweed bio-stimulants can influence yield and profitability and also minimize environmental impacts. However, further trials are needed to validate the findings of this study.

The present study aimed to assess not only the efficacy of sap from two seaweeds Kappaphycus alvarezii (K-sap) and Gracilaria edulis (G-sap) on productivity and quality of Zea mays under rain-fed condition, but also to quantify whether sap application is beneficial in terms of lowering the carbon and phosphate footprint of mineral fertilizers per unit of produce. Field experiment was carried out to test 18 treatments, viz., 5 concentrations (2.5, 5.0, 7.5, 10 and 15 %) each of K-sap and G-sap applied along with recommended rate of fertilizers (RRF); 3 concentrations (7.5, 10 and 15 %) of each of the two types of sap applied along with 50 % RRF; alongside 2 control treatments T1 (water spray along with 100 % RRF) and T18 (water spray along with 50 % RRF). The optimal treatments that enhanced the grain productivity of maize were 5 % G-sap or 7.5 % K-sap applied in conjunction with 100 % RRF and the grain yield enhancements ranged from 21.4 to 29.8 % as compared to T1. Significant increase in P (35.5 %) and K (14.4 %) content in grains was observed through G-sap application, suggesting bio-stimulation in absorption of these elements. Notably, stover yield production at reduced RRF in certain combinations with sap was at par with T1 suggesting a possible saving on fertilizer requirement for fodder production under rain-fed conditions. Compared to T1, there was marked reduction of 17.5 and 23.1 % in global warming potential per unit of produce when 7.5 % K-sap and 5 % G-sap were used respectively in conjunction with 100 % RRF.

Jatropha curcas is a potential biofuel crop and press cake is an important co-product obtained during biodiesel production. In order to assess the long-term effects of application of Jatropha press cake on soil quality parameters and to evaluate its efficacy as an organic manure, a field trial was carried out in degraded wasteland of Gujarat, India with four treatments chosen on the basis of equivalent amounts of N supply through urea, Jatropha press cake, farmyard manure (FYM) and half dose of urea. The activities of various exocellular enzymes involved in nutrient cycling, microbial biomass carbon, respiration, fluorescein diacetate (FDA) hydrolysis and the influence of chemical composition of the organic manures on these parameters were studied following seven continuous years of application of nutrient amendments. Degradation of the phorbols, toxins present in press cake, was also studied. Except for acid phosphomonoesterase, enzyme activities were higher in FYM compared to other treatments. Lignin content was higher in FYM while the ligno-cellulosic fraction was higher by 45% in press cake as compared to FYM. Notably, carbon accumulation in soil was higher per unit of applied carbon in press cake treated soil. Ratio of microbial biomass carbon to soil organic carbon was higher in press cake amended soil. Phorbol esters were degraded to negligible amounts. The increase in yield of Jatropha following improved enzyme activities, microbial carbon and FDA hydrolysis owing to press cake application in erstwhile poor soils unequivocally establishes the scope of Jatropha press cake in reclaiming unfertile wastelands sustainably.