For many centuries manure application to the soil has been common practice. Organic amendments and fertiliser applications can increase crop yields and soil organic matter (SOM). However, the long-term impacts on soil physical fertility are often neglected. This study was carried out on the Broadbalk Wheat Experiment at Rothamsted, UK, established in 1843 on an Aquic/Typic Paleudalf soil. Application of farmyard manure (FYM), N fertiliser and wheat straw on total organic C (CT), labile C (CL) and non-labile C (CNL), total N (NT), mean weight diameter (MWD) and unsaturated hydraulic conductivity (Kunsat) were studied on wheat ('Triticum aestivum') and adjacent woodland and pasture areas. Manure additions, N fertiliser and straw incorporation increased all C fractions, particularly the CL fraction. The addition of 35 t ha⁻1 year⁻¹ of FYM increased CT to 2.5 times that of the control (no fertiliser) treatment and CL to 5 times that of the control. With highest N application and straw returned, CT increased by 1.3 times and CL by 1.5 times that of the control treatment. There were linear relationships between rate of N fertiliser applied and all C fractions, with the rate of increase almost double with straw than straw removed. Manure application improved MWD, as did high N fertiliser additions with straw returned. Application of N fertiliser only increased MWD and Kunsat (at 10 mm tension) if straw was returned, while the addition of manure resulted in decreased Kunsat. The highest Kunsat rate was on the high N fertiliser, straw returned treatments. The uncropped areas all had high soil structural stability. Similar relationships occurred between all C fractions and NT and MWD for the high C soils, but relationships were much stronger with CL than the other C fractions in the low C soils. These results showed that soils with low C concentration are more reliant on CL for structural stability.

Degradation of soil structure can lead to increased risk of run-off and soil erosion, and therefore, it is necessary to implement management practices that are more sustainable and will enhance and rehabilitate soils while increasing food production. The impact of small-grain rotations grown with legumes, fallow and continuously on total C (CT), labile C (CL), non-labile C (CNL), total N (NT), aggregation expressed as mean weight diameter (MWD) and infiltration determined as unsaturated hydraulic conductivity (Kunsat) were examined in a long-term rotation trial established in 1966 on a Black Earth ('Pellic Vertisol') and a Red Clay ('Chromic Vertisol') soil near Tamworth, in New South Wales, Australia. The results were compared with an adjacent uncropped pasture on each soil type. Cropping reduced all C fractions, NT, MWD and Kunsat on both soils, which were further degraded when long fallowing was included in the rotation. CL decreased by 70% with long fallow in the Red Clay and by 78% in the Black Earth compared with the adjacent pasture, whileMWDdecreased by 61% in the Red Clay and 91% in the Black Earth. Rotation of cereals with legumes resulted in smaller decreases in C fractions, NT, MWD and Kunsat when compared with pasture. Rotation with lucerne ('Medicago sativa') resulted in 41% higher CL, 45% higherMWDand 87% higher Kunsat (10 mmtension) than long fallow on the Red Clay soil and 65, 126 and 43% higher on the Black Earth soil. There were strong positive correlations of soil C fractions and NT with MWD for both soil types. Similar significant relationships were found for all C fractions and NT with Kunsat (10mm tension) for the Red Clay soil, but not for the Black Earth. Rotations with forage legumes can limit declines in C fractions, NT, MWD and Kunsat when cropping these soils and has potential to increase soil sustainability.

Increasing concentrations of CO₂ in the atmosphere are placing emphasis on the necessity for sequestering carbon (C) into soil organic matter (SOM). By studying the interior parts of soil aggregates, a better understanding of the incorporation and sequestration of plant residue materials within these aggregates could be obtained. The location of newly added plant residues within soil aggregates may also assist in the investigation of the impact of these newly added plant materials on soil aggregation. This study investigated two different techniques for determining the location of newly added plant residues within soil aggregates by using plant materials labelled with ¹⁴C and ¹³C isotopes incorporated into two different soil types, Black Earth (Pellic Vertisol) and Red Clay (Chromic Vertisol). Both autoradiography combined with scanning electron microprobe analysis (¹⁴C) and secondary ion mass spectrometry (SIMS) (¹³C) were successfully used for detecting the presence and location of the newly added plant residues fragments within soil aggregates of both soil types. The use of labelled plant materials is essential for the study of the location of newly added plant materials within soil aggregates, and this has proven to be a useful tool for studying the impact of residue additions on soil aggregate formation. Furthermore, these methods have been shown to be useful for determining the incorporation and sequestration of C materials within soil aggregates. The development of the ¹³C SIMS technique could alleviate the necessity for the use of the radioactive isotope ¹⁴C in soil studies.

Manure is a source of plant nutrients and can make a valuable contribution to soil organic matter (SOM). Two experimental sites were studied on a Halpic Phaeozem soil near Bad Lauchstadt in Germany. The first experiment, called the static experiment, commenced in 1902. The impact of fresh farmyard manure (FYM) (0, 20 and 30 t ha⁻¹ 2 year⁻¹) combined with P, K and N fertiliser application on total organic C (CT), labile C (CL), non-labile C (CNL), total N (NT), mean weight diameter (MWD) and unsaturated hydraulic conductivity (Kunsat) was investigated. The second experiment commenced in 1984 and investigated the effect of extreme rates of fresh FYM applications (0, 50, 100 and 200 t ha⁻¹ year⁻¹) and cropping, or a continuous tilled fallow on the same soil properties. At both sites a nearby grassland site served as a reference. On the static experiment, FYM application increased all C fractions, particularly CL, where application of 30 t ha⁻¹ 2 year⁻¹ increased CL by 70% compared with no FYM application. Fertiliser additions to the static experiment had a positive influence on C fractions while NT increased from both FYM and fertiliser application. MWD increased as a result of FYM application, but did not reach that of the grassland site. Both fertiliser and FYM application increased Kunsat (10 mm tension) on the static experiment. In the second experiment application of 200 t ha⁻¹ year⁻¹ of FYM increased concentrations of CL by 173% and of CNL by 80%, compared with no FYM application to make them equivalent to, or greater than the grassland site. A continuously tilled fallow resulted in significant decreases in all C fractions, NT and MWD compared with the cropped site, while Kunsat (10 mm tension) was increased on the 0 and 50 t ha⁻¹ year⁻¹ treatments as a result of a recent tillage. There was no difference in Kunsat between the cropped and the continuous tilled fallow at FYM applications of 100 and 200 t ha⁻¹ year⁻¹. There were similar significant positive correlations of all C fractions and NT with MWD on both experimental sites but the relationships were much stronger on the extreme FYM experiment. Weaker relationships of C fractions and NT with Kunsat (10 mm tension) occurred for the static experimental site but these were not significant for the extreme FYM experimental site. The strongest relationship between C fractions and Kunsat was with CL. This research has shown that applications of FYM can increase SOM and improve soil physical fertility. However, the potential risk of very high rates of FYM on the environment need to be taken into consideration, especially since the application of organic materials to soils is likely to increase in the future.