Exclusion of soil macrofauna did not affect soil quality but increased crop yields in a sub-humid tropical maize-based system

Soil macrofauna such as earthworms and termites are involved in key ecosystem functions and thus considered important for sustainable intensification of crop production. However, their contribution to tropical soil and crop performance, as well as relations with agricultural management (e.g. Conservation Agriculture), are not well understood. This study aimed to quantify soil macrofauna and its impact on soil aggregation, soil carbon and crop yields in a maize-soybean system under tropical sub-humid conditions. A field trial was established in Western Kenya in 2003 with tillage and residue retention as independent factors. A macrofauna exclusion experiment was superimposed in 2005 through regular insecticide applications, and measurements were taken from 2005 to 2012. Termites were the most abundant macrofauna group comprising 61% of total macrofauna numbers followed by ants (20%), while few earthworms were present (5%). Insecticide application significantly reduced termites (by 86 and 62%) and earthworms (by 100 and 88%) at 0-15 and 15-30 cm soil depth respectively. Termite diversity was low, with all species belonging to the family of Macrotermitinae which feed on wood, leaf litter and dead/dry grass. Seven years of macrofauna exclusion did not affect soil aggregation or carbon contents, which might be explained by the low residue retention and the nesting and feeding behavior of the dominant termites present. Macrofauna exclusion resulted in 34% higher maize grain yield and 22% higher soybean grain yield, indicating that pest damage – probably including termites - overruled any potentially beneficial impact of soil macrofauna. Results contrast with previous studies on the effects of termites on plant growth, which were mostly conducted in (semi-) arid regions. Future research should contribute to sustainable management strategies that reduce detrimental impact due to dominance of potential pest species while conserving soil macrofauna diversity and their beneficial functions in agroecosystems

Soil carbon and nitrogen cycling following afforestation with mixed-species tree plantings.

Mixed-species restoration tree plantings are being implemented increasingly throughout the world as they may contribute to mitigate climate change, as well as providing other ecological benefits such as increasing biodiversity. However, the response of soil C after afforestation remains unclear. In order to assess the soil’s C sequestration potential of afforested pastures, we need to understand the factors and processes driving C sequestration. This thesis-by-publication consists of four data-chapters, contributing to answering two main questions that are important when assessing the C sequestration potential of afforested pastures: 1) Can including nitrogen(N)-fixing trees species in plantings increase C sequestration? and 2) How will a drying climate affect the relative C sequestration potential of mixed-species plantings and pastures? A meta-analysis presented in Chapter 2 showed no substantial changes in soil C or N across three decades after afforestation of pastures, in the Mediterranean climate of Australia. However, C stocks under remnant woodlands were significantly higher compared with C stocks in afforested pasture land, suggesting that afforestation may have the potential to increase soil C over longer time scales. The selection of tree species is an important aspect when designing tree plantings and may help increase soil C sequestration rates. Soil C and N contents were assessed under N-fixers and non-N-fixers. Overall, there were higher levels of soil C and N under N-fixing trees compared with non-N-fixing trees. At the individual planting scale, the patterns were less clear with both large increases and decreases occurring across the range of sites. The results indicate that the inclusion of N-fixers may help to increase soil C and N, but that the response may be site- and tree species-specific. Differences between N-fixing tree species were confirmed in Chapter 4. Two N-fixers were found to be substantially different in terms of C and N addition to the soil, as well as microbial community composition beneath them. There were also indications that fixed atmospheric N was utilized by the non-N-fixing trees, most likely through tight root connections, as opposed to via the decomposition of N-fixers litter. This indicates that even in dry environments, where litter decomposition is slow, the inclusion of N-fixing tree species can be beneficial in the early development of a tree planting. Carbon cycling in dry environments are largely driven by wetting and drying cycles of the soil. As precipitation frequencies are predicted to decrease, it is important to understand the response of soil C and N dynamics to different frequencies of wetting and drying cycles. In Chapter 5, it was found that while the concentration of soil C was similar in pasture and tree planting soils, respiration was significantly lower in pasture soil. Although there was little difference in the composition of the soil microbial community in any of the soils or wetting treatments, differences in the levels of potentially mineralized N (PMN) may indicate a difference in microbial activity. Cumulative CO2 emission was significantly lower in the reduced wetting treatment compared with the historical wetting treatment. The size of the reduction was the same for both land uses, indicating that land use change did not affect the response of soil to a reduction in wetting frequency. Finally, the findings in this thesis were discussed in the context of the two questions posed above and recommendations for future research were given

Soil carbon and nitrogen cycling following afforestation with mixed-species tree plantings.

Mixed-species restoration tree plantings are being implemented increasingly throughout the world as they may contribute to mitigate climate change, as well as providing other ecological benefits such as increasing biodiversity. However, the response of soil C after afforestation remains unclear. In order to assess the soil’s C sequestration potential of afforested pastures, we need to understand the factors and processes driving C sequestration. This thesis-by-publication consists of four data-chapters, contributing to answering two main questions that are important when assessing the C sequestration potential of afforested pastures: 1) Can including nitrogen(N)-fixing trees species in plantings increase C sequestration? and 2) How will a drying climate affect the relative C sequestration potential of mixed-species plantings and pastures? A meta-analysis presented in Chapter 2 showed no substantial changes in soil C or N across three decades after afforestation of pastures, in the Mediterranean climate of Australia. However, C stocks under remnant woodlands were significantly higher compared with C stocks in afforested pasture land, suggesting that afforestation may have the potential to increase soil C over longer time scales. The selection of tree species is an important aspect when designing tree plantings and may help increase soil C sequestration rates. Soil C and N contents were assessed under N-fixers and non-N-fixers. Overall, there were higher levels of soil C and N under N-fixing trees compared with non-N-fixing trees. At the individual planting scale, the patterns were less clear with both large increases and decreases occurring across the range of sites. The results indicate that the inclusion of N-fixers may help to increase soil C and N, but that the response may be site- and tree species-specific. Differences between N-fixing tree species were confirmed in Chapter 4. Two N-fixers were found to be substantially different in terms of C and N addition to the soil, as well as microbial community composition beneath them. There were also indications that fixed atmospheric N was utilized by the non-N-fixing trees, most likely through tight root connections, as opposed to via the decomposition of N-fixers litter. This indicates that even in dry environments, where litter decomposition is slow, the inclusion of N-fixing tree species can be beneficial in the early development of a tree planting. Carbon cycling in dry environments are largely driven by wetting and drying cycles of the soil. As precipitation frequencies are predicted to decrease, it is important to understand the response of soil C and N dynamics to different frequencies of wetting and drying cycles. In Chapter 5, it was found that while the concentration of soil C was similar in pasture and tree planting soils, respiration was significantly lower in pasture soil. Although there was little difference in the composition of the soil microbial community in any of the soils or wetting treatments, differences in the levels of potentially mineralized N (PMN) may indicate a difference in microbial activity. Cumulative CO2 emission was significantly lower in the reduced wetting treatment compared with the historical wetting treatment. The size of the reduction was the same for both land uses, indicating that land use change did not affect the response of soil to a reduction in wetting frequency. Finally, the findings in this thesis were discussed in the context of the two questions posed above and recommendations for future research were given

Water Stress Scatters Nitrogen Dilution Curves in Wheat

Nitrogen dilution curves relate a crop’s critical nitrogen concentration (%Nc) to biomass (W) according to the allometric model %Nc = a W-b. This model has a strong theoretical foundation, and parameters a and b show little variation for well-watered crops. Here we explore the robustness of this model for water stressed crops. We established experiments to examine the combined effects of water stress, phenology, partitioning of biomass, and water-soluble carbohydrates (WSC), as driven by environment and variety, on the %Nc of wheat crops. We compared models where %Nc was plotted against biomass, growth stage and thermal time. The models were similarly scattered. Residuals of the %Nc - biomass model at anthesis were positively related to biomass, stem:biomass ratio, Δ13C and water supply, and negatively related to ear:biomass ratio and concentration of WSC. These are physiologically meaningful associations explaining the scatter of biomass-based dilution curves. Residuals of the thermal time model showed less consistent associations with these variables. The biomass dilution model developed for well-watered crops overestimates nitrogen deficiency of water-stressed crops, and a biomass-based model is conceptually more justified than developmental models. This has implications for diagnostic and modeling. As theory is lagging, a greater degree of empiricism might be useful to capture environmental, chiefly water, and genotype-dependent traits in the determination of critical nitrogen for diagnostic purposes. Sensitivity analysis would help to decide if scaling nitrogen dilution curves for crop water status, and genotype-dependent parameters are needed

Table_1.DOCX

<p>Nitrogen dilution curves relate a crop’s critical nitrogen concentration (%N_c) to biomass (W) according to the allometric model %N_c = a W^-b. This model has a strong theoretical foundation, and parameters a and b show little variation for well-watered crops. Here we explore the robustness of this model for water stressed crops. We established experiments to examine the combined effects of water stress, phenology, partitioning of biomass, and water-soluble carbohydrates (WSC), as driven by environment and variety, on the %N_c of wheat crops. We compared models where %N_c was plotted against biomass, growth stage and thermal time. The models were similarly scattered. Residuals of the %N_c - biomass model at anthesis were positively related to biomass, stem:biomass ratio, Δ¹³C and water supply, and negatively related to ear:biomass ratio and concentration of WSC. These are physiologically meaningful associations explaining the scatter of biomass-based dilution curves. Residuals of the thermal time model showed less consistent associations with these variables. The biomass dilution model developed for well-watered crops overestimates nitrogen deficiency of water-stressed crops, and a biomass-based model is conceptually more justified than developmental models. This has implications for diagnostic and modeling. As theory is lagging, a greater degree of empiricism might be useful to capture environmental, chiefly water, and genotype-dependent traits in the determination of critical nitrogen for diagnostic purposes. Sensitivity analysis would help to decide if scaling nitrogen dilution curves for crop water status, and genotype-dependent parameters are needed.</p

Effecten van koolstofvastleggende maatregelen op de (BLN)- bodemkwaliteit indicatoren - Deel 2

Om te evalueren welke landbouwkundige maatregelen bijdragen aan koolstofvastlegging in de bodem worden binnen het onderzoeksprogramma Slim Landgebruik sinds 2018 Lange Termijn Experimenten (LTE's) bemeten. Hiermee wordt bijgedragen aan de in het klimaatakkoord vastgestelde doelstelling om vanaf 2030 0,5 Mton CO2-eq emissie te reduceren door vastlegging van koolstof in minerale landbouwgronden (zand en klei). Binnen Slim Landgebruik wordt ook onderzocht in hoeverre het vastleggen van koolstof in de bodems gepaard gaat met het behouden van- of verbeteren van de bodemkwaliteit en daarmee kan bijdragen aan duurzaam bodembeheer zoals verwoord in het Nationaal Programma Landbouwbodems (NPL). Vanaf 2019 zijn de koolstofmetingen in de LTE's daarom aangevuld met bodemkwaliteitsmetingen aansluitend bij de indicatorset van de BLN (Bodemindicatoren voor Landbouwgronden in Nederland). Deze set bevat naast organische stof indicatoren ook fysische, chemische en biologische indicatoren. Hiermee kan het ‘meekoppelen’ van de bodemkwaliteit met de koolstofvastleggende maatregelen worden onderzocht. Vorig jaar is een eerste rapport (Hoogmoed et al., 2021) (Deel 1) opgeleverd met de metingen uit 2019. Voorliggend rapport (Deel 2) beschrijft de resultaten van de BLN-metingen uit 2020. Dit jaar bevat het rapport meer maatregelen voor de veehouderij: leeftijd grasland verhogen, kruidenrijk grasland en wisselteelt mais-gras(klaver). Voor de akkerbouw gaat het om de maatregelen groenbemesters, compost toevoegen en meerjarige akkerranden. De resultaten uit voorliggend rapport (Deel 2) en het vorige rapport (Deel 1) samen geven een beeld van de meekoppeleffecten van BLN-indicatoren bij een gemeten toename in bodemkoolstof. We zien bij zeven LTE's een trendmatige of een significante toename in het percentage koolstof in de bodem. Deze toename in bodemkoolstof ging in alle gevallen gepaard met een toename in hot-water extractable carbon (HWC). Bij zes LTE's was ook een toename in stikstof totaal te zien en in alle zeven LTE's was er een toename in één of meerdere biologische indicatoren. Op basis van de metingen in de LTE’s in zand en kleigronden kan worden geconcludeerd dat de maatregelen die een toename in bodemkoolstof laten zien een neutraal of positief effect hebben op de bodemkwaliteit. Hierdoor kan de voorzichtige conclusie worden getrokken dat klimaatmaatregelen positief of neutraal mee-koppelen met de doelstelling om in 2030 alle Nederlandse landbouwbodems duurzaam te beheren

Sturen op bodemweerbaarheid door toediening van organische materialen : TKI-AF-15261

Tien zeer uiteenlopende organische producten zijn onderzocht op hun vermogen om ziektewering van een bodem te verhogen. In potproeven werd aangetoond dat een aantal van deze producten de ziektewering van zandgrond tegen Rhizoctonia solani en Meloidogyne hapla konden stimuleren. Vervolgens zijn de organische producten onder praktijkomstandigheden in de teelt van aardappel en suikerbiet getoetst. Bij een dosering van de producten conform de bemestingsadviezen waren de opbrengsten van de gewassen vergelijkbaar met een kunstmest bemeste controle en werd afhankelijk van de samenstelling van de producten tot 160, 50 en 200 kg/ha aan N, P en K bemesting bespaard (bij aardappel). De producten hadden onder veldomstandigheden een geringe invloed op de ziektewerende eigenschappen van de grond: Pythium ziektewering was in enkele gevallen verhoogd, maar er was geen meetbare verhoging van ziektewering tegen Rhizoctonia en Meloidogyne. Daarnaast zijn diverse (biologische) bodemparameters bepaald