*4.2. Rainfall Runoff Trial*

The addition of CropUpTM (in conjunction with (NH4)2SO4) improved plant growth, plant cover and, consequently, N uptake relative to the treatments based on (NH4)2SO4 alone. Importantly, a higher uptake effectively removed N from the soil pore water where it

was more susceptible to loss through runoff. The reason for the lower N uptake and lower plant cover for the inorganic N treatments is unclear, but may be a consequence of: (a) the high solubility and mobility of mineral N following the solubilisation of (NH4)2SO4 (i.e., 76.4 g/100 g water; [38] relative to CropUpTM (Figure 1)); (b) the possibility that, in the CULow, CUMedium and CUHigh treatments, some of the NH4 released from (NH4)2SO4 may have subsequently been retained by CropUpTM; and/or (c) the plant was able to extract nutrients from CropUpTM over the growing period.

First, following dissolution, soluble NH4 is highly mobile due to the very low CEC of the sand (Table 1 and Figure 1), and this cation (along with other essential nutrients added to the basal solution) may have been transported with surface-applied water deeper into the soil tray and away from the ryegrass seeds and emerging plant roots. If the availability of N and other nutrients that are critical for seedling survival was inadequate, then poor grass establishment and survival would be expected. This may explain the poor establishment growth and percent coverage of the ryegrass cover for the treatments receiving (NH4)2SO4 alone.

Second, the inclusion of (≈14.5% by weight) zeolite in CropUpTM, coupled with slower dissolution rates, may have enabled the retention of some of the solubilised NH4 closer to the point of soil incorporation, and hence closer to the developing roots. The effect of the added zeolite on NH4 behaviour was not investigated specifically in this study but based on the strong preference of zeolite for NH4 [39,40]. A proportion of the added N may have been preferentially adsorbed and removed from the pore water over the 42-day growth period. If the zeolite retained a proportion of the added NH4 close to the source, and a proportion of this cation was subsequently available for plant uptake, this may have aided higher plant growth in the treatments that received CropUpTM.

Third, plants have strategies to enhance nutrient uptake from the rhizosphere, such as by exuding organic acidic anions (citrate, malate and carboxylates) from plant roots [41–44], by the release of H ions from the roots to maintain cation/anion balance during nutrient uptake [42,45], or by stimulating nutrient mineralisation of the organic substrate [46–48]. The increased availability of one nutrient (e.g., P) by plant root exudates can cause a concomitant increase in the availability of co-precipitated or complexed nutrients (e.g., Mn) [43]. Although the reason for the enhanced nutrient uptake in the presence of CropUpTM was not investigated in this study, other researchers have reported that ryegrass (*Lolium rigidum* and *Lolium perenne* L.) roots can encourage the dissolution of phosphate and other nutrients from sparingly soluble inorganic and organic soil fractions roots due to proton excretion [44,45].

Given the slow release of N from CropUpTM (Figure 1), much of the mineral N measured in the runoff from the (NH4)2SO4 + CropUpTM treatments most likely originated from the inorganic (NH4)2SO4 component, which had not been taken up by the ryegrass during the growth period. Furthermore, the loss from the treatments receiving CropUpTM was not significantly different from that from the control, suggesting that N had been released from CropUpTM at a rate that matched the plant N uptake requirements. This match between N availability and plant uptake is supported by the significantly (*p* < 0.05) higher N content of the ryegrass grown in the presence of CropUpTM (Figure 3). Therefore, the slower release rate of N in the presence of an established plant cover clearly demonstrates the beneficial effect of CropUpTM on increasing plant N uptake and reducing N runoff losses compared with inorganic N fertiliser alone. The higher plant uptake of macronutrients and micronutrients demonstrates that CropUpTM may also act as a slow-release fertiliser.

The nitrogen use efficiency (NUE) was higher in the treatments incorporating CropUpTM, demonstrating the benefits of organic materials in N fertiliser management. The NUE values for the ryegrass that received inorganic N fertiliser only are similar to the NUE reported for the urea N applied to the ryegrass, being 33–47% for N rates of 17–50 kg/ha [49]. The recovery of fertiliser N as determined from the NUE of major agricultural systems in Australia ranges from 28% in sugarcane to 45–62% for irrigated cotton and 78% for dairy [30], compared with an estimated global NUE of 33% [32]. Clearly, the inclusion of

organic materials as an N source can be highly beneficial for increasing NUE in pasture and cropping systems, and represents a key strategy to better manage N in the environment with the intensification of agriculture [31,32,50,51]. Importantly, the inclusion of organic materials capable of supplying plant nutrients may represent one step in the development of cropping systems on marginal soils such as sands.

Our findings extend previous research on the mechanisms involved in the beneficial effects of combined organic and inorganic fertilisers. Several studies have shown that the addition of organic fertiliser to inorganic fertiliser has a positive effect on soil chemical and biological properties [52,53]. Wen et al. (2016) [54] showed that this fertilisation regime increases nutrient uptake via stimulating root growth. It also reduces N leaching and enhances denitrifier activity [55]. Here, we provide further evidence that this combination benefits plant N use efficiency via reduced N runoff losses. This has important implications particularly for increased NUE of farming systems in high rainfall geographical locations.
