B.PAS.0502 - Boosting natural regeneration of nitrogen capital in grazing lands

Summary

This project focused on how rangeland management influences the capacity of biological soil crusts to replenish soil nitrogen lost in export of livestock production. Using multi-tiered cutting-edge science spanning from gene-centric analysis to satellite imagery, the project explored tools to quantify the presence, composition, and function of biocrusts in long-term fire and grazing trials over multiple seasons. Imaging can distinguish biocrusts from degraded bare and vegetated soil. Biocrust-topsoil has four times more nitrogen and carbon than bare soil confirming their essential role in rangelands. N input from biocrusts of 5kg per hectare represents a substantial input that accounts for approximate one sixth to half of the annual pasture N demand in extensive northern Australian ecosystems. Bacterial genes responsible for nitrogen fixation emerged as a sensitive indicator, which responds to landscape condition and season, confirming that management modulates nitrogen fixing capacity.

Stocking rates that maintain good land cover support nitrogen fixation via biocrusts, which are most productive when sheltered by grass. Informed by land condition and climatic conditions, wet season spelling can facilitate nitrogen inputs during peak biocrust activity. Fire regimens using moderate temperatures will speed biocrust recovery while reducing litter buildup and boosting nitrogen fixation. Based on previous research, well-developed biocrusts in healthy landscapes generate annually 5kg nitrogen per hectare. This conservative estimate means that per square km, biocrusts provide 25–50 tonnes nitrogen for pasture with 1–2% dry matter nitrogen. The findings can augment good industry practice by harnessing biocrusts to annually replenish nitrogen in grazed rangelands. Rapid and cost-effective restoration of biocrusts could support the rapid recovery of rangelands after disaster, and there are insights on how this could be accomplished.

Objectives

The key objective was to identify how pasture management affects the capacity of biocrusts to generate nitrogen as natural fertiliser so that producers can maximise the capacity to replenish nitrogen in soil for pasture growth. The project addressed these questions:
• Can image analysis (proximal, drone/UAV, satellite) quantify biocrusts to upscale from patch to landscape?
• Do biocrusts have preferred habitats within landscapes?
• Do biocrusts on different soil types differ in species composition and function?
• How do fire and grazing impact biocrust presence and capacity to fix nitrogen?
• Can DNA analysis of biocrust organisms provide information on nitrogen fixing capacity?
• Can analysis of bacterial genes quantify biocrust processes, especially nitrogen fixation?
• Which management recommendations emerge from the findings?

Key findings

• This project provides unparalleled insight into the presence and function of biocrusts. The principal question of ‘how much nitrogen can biocrusts generate for pastures’ has to consider the net effects of biocrust nitrogen input, uptake by pasture, and nitrogen loss from soil.
• In degraded landscapes that lack vegetation cover and are eroding, nitrogen input by biocrusts is lowest. Such landscapes have negative feedback: low nitrogen input, low uptake by pasture, and high risk of nitrogen loss.
• In well-managed landscapes with suitable vegetation cover, nitrogen input from biocrusts is highest. Such landscapes have positive feedback: high nitrogen input, high uptake by pasture, and low risk of nitrogen loss.
• Nitrogen input is maximised when biocrusts are undisturbed in the wet season during the peak nitrogen fixation period. Current satellite-based pasture assessment can be expanded to include biocrusts and can be refined with UAV imaging where required.
• Biocrust cover on soils preserves and likely increases soil nitrogen and carbon stocks.
• Bacterial communities in biocrusts change in response to environmental conditions, and management can promote biocrusts with desirable functions.

Benefits to industry

The project raised industry awareness of biocrusts so that producers can accommodate biocrusts in their land and grazing management decisions. The benefits are: (i) biocrusts are most prolific in rangelands that are carefully managed for pasture retention and (ii) biocrusts can annually regenerate soil nitrogen to ensure soil health and fertility and support pasture growth for long-term sustainable use of rangelands and livestock production. This requires:
(1) Optimising stocking rates in line with existing and emerging recommendations
(2) Wet season spelling informed by paddock condition, i.e., more degraded paddocks will benefit from more frequent and/or longer wet season spelling.
(3) Considering soil type because biocrusts are more vulnerable in sandier soils than clayey soils, with degraded land on sandy soils taking longer to recover biocrust cover and function.
(4) Optimised fire regimens that reduce leaf litter build up while ensuring speedy recovery of biocrusts to be active in the wet season.
With producers aiming to maximise pasture production, managing biocrusts allows controlling erosion and maximising input of the ‘renewable nutrient nitrogen’ which is quantitatively the most important nutrient, accounting for 60–70% of soil-derived nutrients. A confident estimate is that well-developed biocrusts generate annually 5kg nitrogen per hectare or 500kg N per square km.

MLA action

The project has delivered crucial insights into the long-term sustainable management of rangelands grazing systems. This has important implications for CN30 outcomes and building natural capital outcomes. The project outputs should be showcased at the next NABRC conference as they align with NABRC's highest priority research issue, which is declining land condition trajectory. Further research investment is recommended, but is likely to require producer levies.

Future research

This project took comprehensive steps to examine biocrusts in the context of Australian rangelands and their management. Situating the project at two research stations with fire and grazing management enabled a dual focus on:
(i) developing methods to detect and analyse biocrusts and their nitrogen fixation potential, and
(ii) discerning biocrust responses to environmental and management variables
(iii) develop modelling program (e.g. CLEM) to further elucidate industry benefits
(iv) incorporate biocrusts into land management including natural capital and restoration goals.